First, how you use that back on RB? i mean i know that RB is 6x7, do you use the back on another camera larger than 6x7?
Second, that depends on the quality of the battery itself and also the usage and temp maybe, you are well knowledgable than me, you should know those, i have rechargeable batteries from 3 or 4 brands, the best 2 were Sanyo Eneloop and Pearstone, Eneloop live longer though, i had 4 AA of them on my 580EXII flash since last year and still they have run, i don't use the flash more than once or less per month, but the battery didn't drain faster as some other batteries, Pearstone also have that lie milage too but it drained quicker than Eneloop, maybe another idea is that when you don't use the battery just take it out from the device, not sure that will save the life of the battery power but who knows, scientific people should answer you.