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.
That uncertainty about when a hack could happen gives hackers a great advantage, for example if they have discovered a vulnerability in the code of the casino from which they can get money instead of using it right away they can just wait until they hot wallet is full again and steal more money than if they just tried the hack at a random time.
And if they hackers are even more devious then they could wait until a whale decides to withdraw their money and the casino is forced to move more money than what they are used to into their hot wallet and then steal the money, giving themselves even more money and damaging the casino even more.