DXM is a tricky and fickle substance... DXM from Delsym is DXM Polystirex, meaning it's extended release, so it naturally takes longer to kick in as oppose to DXM in, say, cough gels. 400mg of dxm poly is definitely enough to get you to the second plateau, possibly even light third, but I wouldn't necessarily count on it.
If you take all 400mg at once, I suggest taking it on a moderately - completely empty stomach, that way it absorbs fastest. This may cause nausea and some vomiting, but unfortunately it's incredible difficult to get around that with DXM. If you're unsure about dosages, look up a DXM dosage calculator. That could be extremely helpful.
When you take the DXM, it will take 1 to 2 hours to kick in, and once it does, you won't go from first plateau to second plateau like you said, you'll just steadily dive into your second plateau over the course of about ten to twenty minutes from when you first start to notice effects. These effects are similar to being drunk and/or stoned. Very spacey. You also can get some pretty cool closed eye visuals (CEV).
It usually lasts around 8 hours, but since you're taking dxm poly, it could be up to 12 hours. After that, some people get a mild hungover effect, although that's never been the case with me. I'd call it more of an afterglow. But I usually only get that if I take enough to get me to the third or fourth.
During the afterglow the next day, I've always been able to talk with people normally without arousing suspicion of whether I'm high or not. You should be fine. Depending on how high you are during your dose will determine if you could function around other people, but I'd try to stay away from that. Just sit back, listen to your favorite music, and watch the light show.
BTW, if you ever take DXM from any other medicine make sure it ONLY has DXM in it. NOTHING else! You could end up severely damaging your liver from acetaminophen overdose, or beat your heart to death. But anything with DXM alone is fine. Good luck and be safe!
