I think maximum is around 10 tablets per day, 50mg. You could probably find someone who'd do more for special cases, but that's the ceiling dose usually.
I'd never take over 20mg a day. Did that for a while and it seemed good, but it doesn't last and actually noticed it making ADD symptoms far worse.
My dex hasn't been working so well lately...will be giving it a 6 month break and then reassess symptoms. I might not even need them anymore and I dislike the idea of taking a pill to make me 'normal'
I guess it's different for everyone

top tips IME would be - use minimum effective dose, don't increase dose just because you don't 'feel' it - only increase after getting feedback from those around you on how they've noticed you to be lately (we're pretty crap assessing our own symptoms sometimes) Make sure your phychiatrist is testing you on a regular basis, working memory/coding, IQ test etc. Final tip would be take drug holidays whenever you can - for me that was every weekend and once every 3 months for a week.
Oh, how much am I prescibed

? 20mg a day max, been doing 5mg four times a day (if I remember)
How much are you prescribed? Personally, if I were thinking about going up to 50mg a day I would make sure to try every other treatment beforehand - Strattera + the various formulations of ritalin. Strattera works awesome for some people.