Weird. I'd always heard that Li-ion batteries lose their charge if you don't charge them often. I'm sure they aren't like Ni-cads, or maybe that was li-poly batteries. Anyways. I usually keep my DS charged and use it every day but usually not till the red light. When I go home for a weekend (Living in dorms) I can use it till the red light comes on (because FFIII is fun), and I'm sure I'm still getting 15 or so hours out of it. I've had this Lite since they came out.
*Edit*
http://en.wikipedia.org/wiki/Lithium_ion_battery#Guidelines_for_prolonging_Li-ion_battery_life
Not the best source, but I'll see if I can find a better one later.