Typically, mowers specify 50 - 100 hour oil change intervals.
It is easy to figure out your typical use.
Example: My yard is about 1/4 acre. Takes me approximately 45-60mins to cut it. I cut it every week May through October.
6 months x 4 weeks =24 mows. So... I typically use it less than 25 hours per year. Therefore, I change the oil every 1-2 years.