I must have remembered wrong. Looking back at my sources (GBAtemp's flashcart reviews), it looks like it was about 5 hours at max brightness or 8-9 hours at the second lowest brightness with a retail cart, and about 10% less with a slot 1 flashcart.
Yeah, Li-ion is designed in such a way you do not have to fully use the battery till all the power is depleted.
And about the sleep mode with the R4, it does consume ALOT of power, my DSphat was fully charged, I played for an hour or 3, closed it and put it aside till after dinner, a little over an hour I come back and after just a bit the battery color changed red >.
Mine seems to last about 5 and a half hours(less if I've been watching movies in Moon shell). I rarely ever see that little red light though because I just put mine on the charger when I'm not using it. I play on the second highest light setting though so you could definitely get more time out of the battery on a lower setting. It also helps if you just stick with a certain game instead of constantly resetting or turning off your ds to pick a new one.....