No one knows. Good question though. The problem is in defining "average". The amount of semen a man produces depends largely on how often it is released. While there is generally a small amount made daily that is reabsorbed into the body when not used, those that masturbate daily or even multiple time daily will produce more total volume semen than men that only masturbate occasionally. While sperm production is more of a constant, even then the body has to make room for the new sperm and will either flush it out through a wet dream, or reabsorb the sperm if a man rarely orgasms. There is a large difference in ejaculate by age, region, social income, even factors like sexual preference would need to be included in a study such as this. But the most important factor in ejaculate volume is diet. Men that keep hydrated by drinking plenty of water and stick to a healthy diet of mostly fruit and vegetables produce more ejaculate that than the "average" american man who eats the regular american diet which overtime just clogs everything up. Then we have the supplement like "ropes" that greatly increase ejaculate volume, thereby greatly increases orgasm. Generally speaking, boys in puberty will produce more ejaculate than the middle aged man, in shape men will produce more than couch potatoes, vegetarians will produce more than those who don't pay attention to what they eat, water drinkers will produce more than soda drinkers etc. But in general you can estimate that the regular ejaculate is about a tablespoon worth times the amount of ejaculates for a given group and you can get a ballpark. But defining average leaves the door wide open. It's safe to say it would be a couple gallons or more though.