I have been using the same Ego batteries that came with my first kit back in July 2010. I ordered a second kit at the first of August and have been using all 4 batteries daily since then.
I recharge them daily. Normally takes 40-50 minutes to recharge each one, so I figure I am depleting them to about 30%-40% capacity every day.
Like the Energizer Bunny.....still going. I am past 200 recharges. Your mileage may vary.
You really should have a couple spare batteries and some spare atomizers as well. When they go, they go. Then where are you?