We service lawn equipment. You should change your engine oil every 25-50 hours. At 50 hours, this this the same running time as 3,000 miles on a car. Cars run on nice clean, paved roads. Your mower runs on a dusty lawn. The more dust, the more often you should change your oil and oil filters.
The #1 killer of small engines is dust. If you're wondering how often you should change your air filter, know that if you're having trouble breathing with the dust in the air, your engine is doing the same. There is no time frame to keep in mind when thinking about changing your air filter. You need to check and change it according to the conditions you're mowing in.