IT really all comes down to the gear. That's what most newbies just don't appreciate. It's the only completely artificial drug on the planet, and so it's different each and every time. Most stuff out there is going to be pretty consistent with the above posts...
however....find some really really well cooked stuff...like I get...and it's a whole 'nother ball game...
I consistently get not only near pure gear, but insanely well made...
I'm nearly 30...and go through my periods with different substances...and at the moment I regularly smoke crystal every weekend...as this stuff is too good not to enjoy...
with the purer gear...You rush a HELL of a lot more easily/sooner, of a lot less....and for a lot longer...
I'd say 20 mgs of the REALLY GOOD stuff...will have you rushing pretty soon...for a good 5 to 6 hours...but with really clean stuff, I don't get the extra 12-48 hours of pain....in fact, it pretty much just wear off really really nicely...and I never get much of a comedown..
I do take a lot of magnesium, drink a huge amount of water...but the gear is the main thing. I tried a different source a few weeks back, and straight away felt those nasty after affect for a day or two after...
Also...knowing how to actually smoke the stuff properly makes a HUGE difference in the duration, quality and ease in achieving the desired 'rush'...
Sorry if I'm rabbiting on...I must have done 20 mgs around 5 hours ago...and still rushing extremley well.
I find what normally causes the added time of undesired affter-affects, is the simple fact your smoking something dirty...
I can stay awake for two days easy...and as soon as I want...I lay down and crash out with no problems at all...and don't have any of the nasty 'tweakness' afterwards....but only with stuff that's cooked by dude's with real scientific knowledge...