I have not won more than $250 before and the withdrawal were very fast in each. The amounts are also significantly less than $250 in most cases but I am just using what I experienced during withdrawal to post.
The withdrawal can be between some seconds to few minutes depending on the coins.
I think it depends on the amount, more than anything. Withdrawing $250 dollars is not going to be met with the same level of scrutiny as with $250.000 dollars, right? The casino needs to make sure they are giving the money to the right person, and that that person is allowed to even make the withdrawal in the first place. The casino, if the get "lucky", might discover that the player in question violated a rule from the terms of service and is not eligible for making any withdrawals. This seems to be the case, usually, when I look at the people complaining on the forum about winning large amounts of money yet not being allowed to withdraw.