Thanks to everyone for the advice & info. I was thinking it had to be like any other rechargeable item, that they'd lose some "oomph" over time if not used/recharged regularly. But that'd be an interesting to-do item for my housesitters, right? "Water plants, check mail, recharge sex toys..."
I am keeping my fingers crossed, I've had some rechargeable AA batteries (NiMH) that weren't used for ~4 yrs & they worked just fine after a full charge. Granted, they took a bit longer than usual to charge up again!
I definitely agree it'd be nice if some of them had removeable battery packs like cameras or power tools. Heck, the toys that use regular AAs are nice to use with rechargeable AAs, but the Fun Factory G4 toys & Jopen sure don't have removeable batteries. Booo!