The answer may be different than what you are expecting. Casinos change dealers every 20 minutes or so to keep them focused and refreshed. Since casinos are fast-paced businesses, it makes sense to continually rotate new dealers to make sure games are being handled properly.It is very rare, in fact, for dealers to be changed if they are losing too much. As long as no cheating is going on, there is no purpose of changing a dealer because a player is constantly winning at a game.