Well, it's kinda hard to say because it will vary on how you ride, but for estimating purposes let's say you are riding hard you will use a ratio approximately between 40 and 50:1, which would be 2-3oz. of oil per gallon, so lets say you are using 2.5oz per gallon and let say you fill up at 12gallons, so that would be 30oz per fill up. So that about 4-1/4 fill ups per gallon, so you would be adding a gallon every 4-5 tank fulls. Like I said this is pretty heavy riding, because I don't think I use this much oil, I'll use 1-1/2 to 2 gallons a season.
BTW you are using the wrong oil, you should be using a full synthetic, API-TC rated oil, Evinrude oils is TC-W3 rated, it is a high performance oil but it's no ash oil, you should be using a low ash oil.
Lou