On a lawn mower it's recommended to change the lawn mower engine oil once per year or once per mowing season.
I always change my lawn mower engine oil after the mowing season ends and when I have nothing else to do in the winter.
I go through and sharpen my mower blades, change the oil and tune the lawn mower up during the fall and winter months so that the mower is ready to go in the spring and summer for mowing the lawn.
Sometimes you might only need to change your lawn mower oil every 2 years if you do not do a lot of mowing but once per year is a good idea to keep the lawn mower engine protected.
Of course if you do more mowing such as on a daily basis then you might once to change your lawn mower oil ever few months or twice in the mowing season.