Most lawn mower manufactures recommend you change your lawn mowers engine oil every 20 to 50 hours of use.
But a good thing to do is change the oil once per year during an annual tune up before the mowing season starts again.
Keeping your lawn mowers engine oil changed can help extend the life of your lawn mowers engine so once a year is usually good enough if you don't do a lot of mowing.
If you're mowing for a job doing a lot of lawn mowing during the summer you may need to change your lawn mowers engine oil more often before the mowing season is up.
If you do a lot of mowing you should change the lawn mowers engine oil at 20 to 50 hours but if you're just mowing your own lawn during the mowing season then once per year is certainly enough.