How long is a piece of string. The battery life depends a lot on what's being used and when. The Li-ion BP511 doesn't have any memory so you can leave it in the camera and then top it up before you use it. I wouldn't go out on a shoot without checking the battery. besides it only takes about an hour or so to charge it up even if it's well down. If you are in doubt by a spare,you can get them very cheaply nowadays.