Depends on the specific battery. I've looked into rechargable batteries and a big factor in how long they last is their ability to hold a charge. 99% of all rechargable batteries start loosing charge the moment they complete the charging cycle - for instance if you charge up batteries, then go to use them a couple of weeks later, most will be completely flat again. Some are so bad they loose all of the charge in a day, and they get worse the more charges they have.