FFS, I swear I'll make a php script or something to determine various battery life calculations... anyway:
Depends on whether your using expansion pak or jumper pak - jumper, for simplicities sake, draws about 1Ah, expansion 1.5Ah. (On 3.3v) - If you use the pth, it's about 90% efficiency, so if you also add in the screen + 12v line of the n64, It'll be ROUGHLY:
Jumper: 1Ah/Hour, so 3000mAh @ 7.4V will work
Expansion: 1.25Ah/Hour, so 3750mAh @ 7.4 should work.