Depends on the battery, trust me, go read the battery info thread in the stickies, the last like 5ish pages should give you all you need. Drozd's posts in there have serious breakdowns on longevity and such.
As far as 3.7v for the big chuck, it takes 1 18650 battery for that, or 2 cr123 3.7v for 7.4, or 2 LiFePO 3v batteries for 6v. Little takes 1 cr123.
Durations are really based off the mAh (milliAmp hours) rating of the battery itself. Big chuck 3.7 will last you over a day, and up to 2 days with the 3000 mAh 18650 people have reported.
Anyways, like I said, there's a wealth of information in that thread which will help you on where, what, and how many.