Battery life is not fixed - I've had many motherboards and never had to replace a single battery - a friend had to replace a battery after less than 2 years.
The first indication is that the system fails to remember time and date and reverts to the BIOS basic settings on boot-up. If the battery fails, its normally just a matter of replacing it. Some older PC have mobos with the battery soldered into place, but even these CAN be replaced. But the modern ones take watch-type batteries you can pick up from any PC repair shop.