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Well usually casinos offers instant deposits and you can start playing right away.They only want a few confirmations before withdrawing and this amount is usually 3 confirmations.In case you win you just wait for the 3 confirmations and then can withdraw instantly.
As far as I know, in order for you to get and be able to "use" your withdrawals, you will be needing more than 3 confirmations from the blockchain of bitcoin. As I understand it, it is different from instant deposits in which most of the time 1,2, or 3 confirmations are enough to have the deposits credited to your casino wallet.
^ As I remember correctly because I don't have enough knowledge on technical stuff on BTC transactions. The 3 confirmations consider as safe and can not be reversed but this is only for small transactions like $1k below. However, in large transactions like millions of dollars in BTC amount will need six confirmation, this is the standard for most transactions to be considered secure. It would be love to use a gambling site that offer for free withdrawal fee and I hope all of them are generous enough to have that free fees upon withdrawal.