It's 3 to 5 hours for 3DS games and about 5 to 8 hours for DS games.
That's a pretty huge step back.
I remember my Fat DS easily pulling through 12 hours at launch, but I could be remembering wrong, what do you current DS enthusiast think of the battery life?
Also, it would be pretty cool if Nintendo allowed all USB charges to charge the 3DS, kind of like how the Dualshock 3 and PSP works. That might actually take away some of the stress from the shorter battery life.
Edit: I'd also like to mention that it takes about 3.5 hours to get the 3 to 5 hours charge, which is extremely high in comparison to other portable devices. This piece of info was also from Gaf, I can't find it on the link (it's a messy page).
Bet with Conegamer and AussieGecko that the PS3 will have more exclusives in 2011 than the Wii or 360... or something.