I'm probably not a heavy vaper, probably average. An Ego 650mAh battery lasts me at least most of a day. I am using a 1000mAh passthrough battery at the moment, and have been vaping on it for 2 days now. Will probably have to recharge it soon, but I'll just switch to my charged 650mAh one. A plus with the Ego battery, all your 510 atty's, carto's, etc will thread right on to it.
Edit : It's bigger than the 510, but not much bigger diameter than a fountain pen.