Well perhaps leaving your crypto on a gambling casino wallet is still good for a short period of time like a day or weeks, beyond that --that is too risky.
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).
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.
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.