This depends on the casino you choose to play at, along with your typical behaviour at the site. For example, if you generally make withdrawals of, say, £100 and then, out of nowhere, make a £20,000 withdrawal, the casino will take some time to carry out a few additional checks. As mentioned above, larger withdrawals may also trigger an 'enhanced' KYC check where you may need to provide proof of where your money came from. This, however, is just a UK regulation at the time of writing.