my 10 yr old Toro recycler with Tecumseh motor has 25 hr recommended oil changes (per owner's manual).
Since I mow between 40 and 50 times each season I change the oil halfway through my mowing season. I also change it at the end of the season and again before the first mow in the spring. Overkill, I know..but 20 oz of oil (per change) is cheap and I actually enjoy doing it. I also change the plug and air filter each spring, along with sharpening the blade and lubing the rear wheel gears.
The deck will rot and the wheels will fall off before the motor goes!