If you want to know how long the battery will last in your BT device, that depends on the device. Usually the manufacturer will have that information available on the Net or the package.
If you want to know how long the iPhone battery will last with a BT device connected, that's unfortunately an impossible question to answer. Too many variables. It the device active or on standby? Are you using the iPhone for music? How much? Movies? How much? WiFi? How much? How many calls are you making? See what I mean?
I'll tell you that with "average" use I need to charge my iPhone every other day. I'm WiFi connected frequently, listen to music (in the car) about 15 minutes per day, make about 20 minutes of BT calls per day, and watch no movies.