When do you change the oil on a lawn mower?

It all depends on the severity of it's use and the amount of time it is used. I only use mine once every three weeks or so depending on the weather. And I only have a small yard. Less than half an acre. So, I only change mine once a year. If you mow more often, in extremely hot conditions or mow very high grass that works the engine harder then I would change it at least two or three times a year. Of course if it is used professionally then you'll need to change it much more often.

The recommended oil change interval is every 25 hours of use or one year, whichever comes first.