Battery is there for GPS memory of last GPS location. If you turn on your Aircraft and it seems like it is taking forever to get a lock, even though you are flying at the same location, it may be time. When I had mine torn apart for repair, I thought about replacing it then, but I didn't. They are soldered in, so you need a replacement with solder tabs on it.
I have not seen anything written by Yuneec on how long they should last.
Did you happen to see the numbers on the battery to tell the type it is?