It's really hard to say how long a battery will last a person. It depends on how often you're using it, how long of a drag you take etc.
Some people get days off of the big batteries like I use (18650 w/ 2200 mah) I go through 3 batteries per day on average. I'm a very heavy vaper though.
It also matters if you're using low resistance stuff 2 ohm or so, vs standard resistance 3 ohm or so. SR will make a battery last longer.
Also, since you're using a variable voltage device, the higher the voltage setting, the lower the battery life will be.
I'm trying to remember what a typical usage is for a 650mah, I think it was roughly 6 hours and approx 300 puffs, but I may be wrong. I'd have to do some Google or forum searching to be sure.
#1 I have a twist and having issues with keeping it charged, seems like my clearos drain it within a couple hours. Heard it was supposed to last at least 8... I got the 650 mah, so maybe I should have gotten the 1000? Cartos don't seem to do much better, nor do the eGo-C atomizers. Did I get a bad twist? Or am I getting lots of defunk cartos/clearos. I've gone through at least 6 of each and I'm getting really disgusted with it all...