The cookie is there until it is deleted or overwritten in most cases.
In a perfect world all players that signed up under you would always be yours. But let’s say your player signed up for casino a as a play money player, then casino a sends a email which if they deposited, you would earn BUT in that email they throw in banners for casino b and c, in most cases you just lost that player to casino b which didn’t have your affiliate link.
I may be wrong but that is my understanding.