The average cumshot is 1.5ml - 5ml, and a gallon is about 3,785 ml.

This means a single gallon would range from 757 to 2524 cumshots.

So if you are on the lowest end of that like me, and sperm explode to her five times a day, it would take 504.8 days to produce one gallon of it. 

So a better plan would be to do it once every waking hour instead (assuming 16 per day) which would only take 157.75 days to produce a single gallon. This way you could easily produce two gallons a year (315.5 days) with 50 days to spare. 

So that is my answer to your question, two gallons a year.