with the 900mAh Jessy x, I get about 24 hours if I'm working 8-10 hour days (I vape at work, but less), more like nearly a day (say from noon one day until mid morning the next) if I'm just sitting around vaping and working online.
YMMV. But in general, it seems more efficient than the equivalent in "standard" 510 hardware. (With a 1000mAh eGo batt, I get between 10 and 15 hours, generally.
Best,
Ande