What I've found with eGo's as far as total battery life goes, is when they're about to give it up, the led button will fade on activating it.
Now that you mention this, I realize I didn't think as to whether or not the OP was talking about vaping lifespan or operational lifespan, so I shall have to clarify my statements/thoughts
Vaping lifespan(as in how much you can vape between recharges): I have an eGo-T 650mAh battery. From full charge to sleepmode, I'd have to say around 8 hours, and I vape quite a bit(former pack a day smoker)
Note: All batteries degrade over time/recharges, so vaping lifespan will reduce, but will take years before it would be noticible
Operational lifespan(how long the hardware lasts): God only knows. Could continue to work for a month, could work for a decade
Sorry for any confusion I may have caused, I claim derp-moment