Fuel filter changes as often as needed (especially with today's [censored] gas)...lower unit oil annually when you change the water pump impeller.....Penzoil synthetic blend in the oil tank. Also make sure to use a fuel stabilizer to help the above mentioned gas not turn bad as quick.
As for the hours.....highly unlikely that you are actually running the motor 50% of the time that the electronics are being used. In an 8 hour day of using the graphs, I might use the big motor for 15-30 minutes. More time if I'm idling through a stump field, and less time if I'm just moving from a spot to a spot.
Eat. Sleep. Fish.