I have two Swan Crystals which get about 22 hours or so (on one measly AA). One has a Mashmods light in it, so knock off a few hours on that one.
I have two Neo Geo Pocket Colors which get something like 47+ hours on two AAs (that's continuous, I have no idea what it is in spurts). The NGP has, by far, the best battery life of any handheld that you'd actually want to play. Simply amazing. Supposedly using lithiums will increase the life by %50 or so, but since lithiums cost %400 as much, that's not really worth it.
Then there is my GBA SP. I think its maybe 14 hours or so. I have no idea. Nothing compared to an NGP, but considering how much more powerful the GBA is, its pretty good.
GB Micro: Maybe 20 hours. Probably not that much.
DS: 15 hours so so, maybe.
So I'd say about 7 days total, the vast majority landing on the NGPs.
Its interesting to note how as some things advance, some things go backwards. New handhelds obviously aren't meant to be taken very far from a wall outlet, but the 90s ones were pretty amazing. It kind of reminds me of how certain models of Minidisc player could get over 100 hours of playback (using the internal rechargeable and the add-on AA adaptor) where as getting an iPod to last more than a few trips to work and back between recharging is impossible.