When should you stop? When you no longer enjoy it.
When should you give up those weekend-long non-stop gaming sessions? When they get in the way of the rest of your life, and if you fail in the rest of your life, you won't be able to do those weekend-long non-stop gaming sessions.
IOW, don't forget about the rest of the world. As you get older, it's harder to keep the ROW out of things.