I do a 45-minute dice-rolling workout before each session, making sure my rolling hand stays in shape. The last thing I want to do is sprain my wrist rolling fireball damage!
Seriously, though, I only spend a sizable amount of time during character creation, making sure I know all the rules, etc. Once the game session I'm all set and need hardly more than 5 minutes. The most I do is look up spells, now that I'm playing a wizard, but it's not like I don't know pretty much what every spell does already (the regular PHB list, not weird stuff from other books, that is).