MuchBetter deposits and withdrawals are considered instantaneous. This doesn't always mean that everything moves both ways immediately, but it does mean that MuchBetter itself is not the bottleneck.
Deposits are basically always in your casino account in matter of seconds. Usually when you get back to the casino lobby, the transfer has already been completed.
Withdrawals depend more on the casino itself than on MuchBetter. After the casino has processed your withdrawal and actually paid it, the money should show up in your account within the same day.