For a petrol lawnmower that is completely new, the first oil change should be carried out after the first 6 hours of operation. After that, it is sufficient to change the oil once a year, provided the lawn mower is not used excessively.
Here you can see exactly how an oil change works on a petrol lawn mower.
However, if the mowing conditions turn out to be more difficult and the engine is particularly stressed, the oil should be changed more frequently. The rule here is that the oil should be changed after about 25 hours of operation to prevent major damage to the machine.