2 different batteries in carmeras

I know I have the main battery but there is also a little battery in the battery compartment (looks like a large watch battery). how often should this be changed? everything works fine but I was just wondering???

Replace it when it goes flat. It is there to keep power to the date/time and other settings stored in the camera, so they don't reset when you take out the main battery. If you find that your settings reset then change the battery.

I wouldn't worry about when it might go, I have had several DSLRs and never changed the little battery in any of them and I have used the cameras for many years (20D - used for 8 years before selling, 5D - 10 years old and still on the original battery, 40D - 7 years old still happily shooting and a 5Dmk3 which is now 3 years old). So, I may have to change the button battery at some point but I really don't think about it as being an issue.

ok, sounds good. I just wasn't sure.