Just googling "DSi battery life" gave me this:
• Battery Life
Nintendo DSi: The lowest brightness (9-14 hrs), low brightness (8-12 hours), medium brightness (6-9 hours), high brightness (4-6 hours), highest brightness(3-4 hours)
Nintendo DS Lite: The lowest brightness (15-19 hrs), low brightness (10-15 hours), high brightness (7-11 hours), highest brightness(5-8 hours)
Which is much better than the 3DS estimates, which are at 3 hours on low brightness. I wouldn't be so optimistic if I were you.
I'm waiting for the lite.