The problem here is that you don't know when a hack will happen. So, it doesn't really matter for how long you leave your cryptos on the casino. But, surely, the shorter the less risky! (not your keys not your money).
You must always assume that when playing in your casino, any violation of their TOS will get you in trouble of course if you happens to play in a scam casino you are surely going to lose your money
The goud thing, though, is that the money used for gambling is supposed to be a money you can afford to lose and you are supposed to be aware of the risks involved.
The most important thing is you enjoy the game and you have a good feeling after the game win or lose because this is what casinos are offering to their players
On the other hand, I believe securing customers funds is mainly the responsibility of the casino. The least they can do is to have a hot/cold wallet setup. This way even if they get hacked most funds will remain safe and it won't result in their bankruptcy.
They will lose loyal players if they cannot secure the funds of their players, there's a lot of risk playing but players can play comfortably knowing if they won they will get the money.