Well, that depends on the situation and the amount lost. Let's say I got a good amount of wins and decided to get more greedy and then lost everything eventually, including the capital. That could be hell for me, and that'll take at least a couple of days, if not a week.
With regular losses, which I have anticipated before I go gambling for the day, I can say, I can forget it just a few hours after leaving the casino. I think most gamblers do. The only time you will have a hard time forgetting a loss is when you decided to become greedy and lost it.