Figure you are burning at a rate of about 50:1 like most Carb outboards do . I dont know how big your tank is on the boat or motor but say you had a 25 gallon tank every time you empty it you would use a half gallon of oil.
Trep, the bigger v6 motors like my 150 you dont fill the oil on the motor itself. They use a remote mounted tank in the boat that the oil goes in. Its something to the tune of 3.5 gallons that it will hold. I can check my oil level by just looking in the battery compartment at the tank. If i had to fill a small saddle tank i would be filling it constantly, with the big remote tank i fill it just a few times a year( i burn a lot of gas).