Removing your motherboard is one of the most cumbersome tasks when it comes to computers. At least it is not very difficult. You will, however, have to plug out all the cables, remove your graphics card, and unscrew all the screws holding it in place. This will take you at least 20 to 30 minutes, so be prepared for that.
With a light press on the retaining clip, the battery can be removed and replaced. Most newer motherboards use a CR2032 battery, which can be bought for just a few dollars. But there is also the possibility that your board uses a different type, so you should definitely check this beforehand.
The battery only drains if there is no power at all going to the mobo. So if a PC is in use, the battery doesn't drain. How long the battery lasts depends on the quality of the battery. The mobo original typically will last many years. The cheap ones I get from ebay to replace them don't feel like they last as long, but I never measured it.
But even the most reliable battery only has a certain lifespan. Over time, a CMOS battery’s performance can degrade and, in the worst case, can up the ghost before your motherboard is ready to say goodbye. If that happens, the CMOS memory chip can no longer hold your BIOS settings between power cycles, resulting in a full BIOS reset.
Finding the CMOS battery on your motherboard is half the battle: It looks like a small silver disc and is usually positioned next to the main PCIe slot. With a light press on the retaining clip, the battery can be removed and replaced. Most newer motherboards use a CR2032 battery, which can be bought for just a few dollars.
Anywhere from 3-10 years depending on how long the system is sitting unplugged and how lucky you are. I can count on one hands the number of CMOS batteries I've had fail on me, it generally isn't something that you need to worry about a lot of the time. If it fails while the system is in use, absolutely nothing.