I have ADHD and take dexedrine daily, I have been taking a consistent dose of 40mg every day and I have not noticed any tolerance whatsoever in the past two years.
However, people who are advising 100mg, you either have crazy tolerances or your brain don't work right. I took double my dose a couple of times (80mg, and I split that into 40mg x2) and it was very unpleasant. It gave me all sorts of side effects that I never have normally, and there was no euphoria from it. I actually get euphoria from taking my regular dose of 40mg, not higher (and I've been on it for 2 years).
When I first had dex, I tried 20mg all at once, and I was buzzing for the next 10 hours or so, but again it was not a pleasant euphoric feeling and felt a bit dirty, however when I added 0.5 of Xanax it was absolutely incredible (from a recreational standpoint). I had to work my way up to 40mg daily, and that took about a month. Whenever I dose higher the side effects start piling on.
For a first time user id strongly recommend using 10-15mg and then redosing around an hour or two later with a top up of another 10-15mg. Top up doses of dexamphetamine are actually very pleasant and add to the euphoria in a good way rather than a negative. It also gives you a chance to judge whether or not you really need to redose, but even still I'd redose 5mg. Less is actually a lot more with d-amp, when you go into the higher dose ranges it's actually dysphoric, although everyone is different and it may affect you differently.
Oh. And once you've tried dex, adderall will seem like the worst drug in the world. I have a post here from when I dosed 60mg of the stuff and I thought it was a fake pill id taken, and people genuinely thought that I was trolling around, when I was actually freaking the hell out. L-amp is absolutely horrendous.