The individual advertisements themselves are not very energy intensive, so your battery-powered beacon will likely perform consistently until the point that you no longer have enough energy potential in the battery to advertise at all. And then -- ack! -- it's dead.
One of the reasons our RadBeacon USBs have been so popular is that they can be plugged into any USB power source -- a standard USB power supply, an Aerohive AP with an available USB port, an existing POS system, etc. -- and the need to be concerned and to maintain batteries goes away entirely.