Wow. There's so much bad advice already in this thread that I can only reiterate the good advice given: START SMALL. By small, we mean less than 100mg. That means no more than five Robitussin cough gels. More importantly though - be sure you aren't taking another medication that is contraindicated! DXM is best taken alone. When mixed with other drugs, the usual result is negative (of course there are exceptions, but the first time is not the time to explore this area). At worst, being ignorant of the contraindicated drugs can kill you. For example, mixing DXM with an MAOI can kill very easily, and has done so many, many times. Mixing it with other common, seemingly benign meds like tricyclics and/or SSRIs can also be problematic, and though rarely fatal at low doses, will certainly be a horrific experience.
So first step - take an accurate inventory of your current/recent drug use and research the interactions with DXM online - even right here on bluelight - before taking ANY.
As far as taking 600mg - I cannot comprehend why anyone would give that advice to a DXM virgin. It's like taking your 110 lb girlfriend out to the shooting range for her first experience firing a pistol and giving her a Desert Eagle calibrated to .50 - just a bad, bad idea.
To answer your question about plateaus:
The most important thing to understand is that DXM is a very different drug at different levels. In other words, the 2nd plateau is not just a more intense 1st plateau, and so forth. Moreover, with each plateau the differences increase significantly, the intensity increases at a geometric rate rather than a linear one, and the dose/response curve becomes more acute as one progresses to higher and higher doses.
To clarify: If a user of average health and constitution experiences a 1st plateau result from, say, 150mg, he might be able to increase certain aspects of that plateau by taking 200-250mg next time (of course, this might notch him to the 2nd plateau as well - another reason to progress slowly). Now, lets assume that this user requires 275mg to enter into the 2nd plateau. This means, with DXM, that the difference in the experience between a dose of 250mg and a dose of 300 mg will be MUCH more dramatic than the difference between a 100mg dose and a 200mg dose. So it is possible to increase your dose by 100mg and change things less than increasing a higher dose by only 20mg, which is just over ONE COUGH GEL. So the importance of knowing where your plateaus start and stop is of paramount importance.
About the plateaus themselves: Unfortunately I must disagree with almost everything offered thus far with regard to plateaus. The simple fact is that anything beyond the second plateau is very difficult, if not impossible, to describe to a DXM virgin. For the sake of answering to the best of my ability, I submit that the first plateau consists primarily of general giddiness, mild dizziness (not disorientation), and general mood elevation. The most obvious trait that marks the entrance into the 2nd plateau would be music euphoria. By that, I mean that whatever music you listen to will physically sound different. As in substantially different. In most cases of 2nd plateau this difference manifests primarily as absolute fidelity and clarity, what I like to call sonic holography. The technical term is in fact "music euphoria" because for most people this experience is exactly that - euphoric. It's almost an empatheonogenic relationship with the music itself, if that makes any sense. There are other differences too, such as the sensation of sudden, significant insights into things like relationships, physics, the natural world, and self explorations that for the most part are pleasant, though at the higher end of the 2nd plateau some of this can be challenging. Crying is not uncommon.
Third plateau can be induced with an addition of as little as 20-40mg to your 2nd plateau dose. So again, if 300-450 yields a pleasant psychedelic, kaleidoscopic, musical and spiritual experience, then 480 or above can produce effects that are NOTHING like the previous. The most prominent feature of a 3rd plateau experience is the predominant anesthetic effects and the resulting disassociation. If you have never experienced true disassociation, then it is quite possible your first time could be quite scary if it wasn't what you were expecting, and at the dose required to get to the 3rd plateau, you're gonna be there a while. The poster that suggested a DXM trip will last 4 hours at most should be ignored. It's completely inaccurate. The fact is it can easily last ten to twelve hours, though on average (in the user we're imagining here for example) I would propose eight hours duration before return to near baseline, and a night of sleep before total return to functionality/normalcy.
Fourth plateau is so often misrepresented on this board that I hesitate to submit my opinion in detail, except to say that the most prominent feature is the belief that you have died. Please understand, I don't mean the fear that you might die...that can happen at any dose on DXM...I mean the bona fide, 100% experience and ACCEPTANCE OF THE FACT that you have died, and are now (whether willing or not) experiencing a new consciousness entirely. For me, the chief difference - and there are many so this is entirely subjective - would be the lack of linear syntax. In other words, you don't think in words. Words vanish. Attempts to think in words result in bizarre sounds generated internally, as well as severe synaesesthia, timelock, spacial disorientation, amnesia with regard to haven taken a drug to begin with, astral projection, time travel, etc.
In other words, it isn't like shrooms or acid at this level. I know a lot of people are gonna probably flame and disagree on this, talking about their 1200mg trips and shit like that. Fine. But for me, anything over 900mg (three bottles of cough gels at once or 60 gels) will land me squarely in that fourth plateau. You need a sitter for this, period. Experience doesn't matter at the fourth plateau, because of the syntax distortion and amnesia. Each fourth palteau experience is its own singular experience, with very little similarity to the last other than the intensity and duration, both of which are severe.
As a side note, I once tried two of the Zicam (540mg each) at once when doing some spiritual exploration and it was by far the most difficult drug experience of my life. Not something I care to repeat. When I told a friend and fellow dexhead about it, he immediately went and tried it himself and wound up in the ER. No physical harm from the drug, but his behavior caused his sister and mother to panic and they took him in. He remembered nothing about the real world during his trip except waking up in the hospital, where he thought he had been sent as punishment, and represented hell, where they were preparing to amputate his arms and legs and scoop out his brain...and this is after he regained enough of his faculties to understand he was in a hospital...point made I assume?
So don't listen to all this advice about heroic doses. My first inclination is to call bullshit on ninety percent of the stories of 900 and above doses all at once because even with a tolerance this is a huge dose that will generate a seriously intense experience. For reference: I am six feet tall, about 200lbs give or take depending on condition, physically very fit, with two decades plus of psychedelic experience. I've taken DXM daily for months on end on several occasions, building a tolerance to the point that I would take 600 - 900 per day, spread throughout the day in 300mg doses. Even then, any single dose over 750 was enough to push the envelope of tolerability, and 900 has NEVER failed to induce a full fledged fourth plateau.
There exists in DXM lore talk of a plateau Sigma. By all accounts it sounds like a brain damaging dose that simply induces delerium, which is in no way remotely similar to tripping.
Hope this helped more than sounded preachy. Sorry for the length - I just don't want you to miss out on what can be a very pleasant and useful legal drug just because you followed some bad advice.
PEace