I've had my PSP-2000 since late 2007, generally run games off the memory stick (with a UMD in, every time the PSP jiggles even a millimeter, the XMB pops up and asks if I want to quite the game. Seriously, Sony? Seriously?), and get maybe 2-3 hours these days.
I always keep the brightness on the maximum setting unless I'm in a very dark place. I keep the volume at one tick above zero most of the time -- one thing handhelds haven't been very good about is making it possible to have low volumes that the user, but not anyone standing nearby, can hear. It's either mute or blasting, it feels like.