I thought they got their money almost immediately. If that doesn’t happen, then my idea that the money is safe is wrong.
I personally know of a couple of smaller banks which had failed, in which the banks were closed on Friday, and re-opened for business as usual on Monday, after having been merged into other healthier banks. If that happens, then the customer would not have any interruption of access to their money.
If FDIC doesn’t pay people right away in case of a bank failure, then the FDIC insurance would be meaningless.
If a bigger bank takes over the smaller, sure, the money has been there, but if the bank simply goes out of business it has taken a few days to a few months. It is one reason I keep multiple accounts at various banks to limit risk, such as bank computers that go out over the weekend and debit cards have failed to work.