Thirty minutes if you won while watching.
Twelve hours if the auction ended while you were away — counted from the first time you open it, and never more than twelve hours after it ended.
Why thirty minutes is not harsh
You are there. You meant to buy it. And the seller is still at the table with the next item in their hands, deciding whether to put the thing back on the shelf.
A long window sounds kind and is not. It leaves the seller waiting on somebody who has already moved on, and it leaves the under-bidder — who wanted it and was willing to pay — with nothing.
Why twelve hours for a timed auction
Because you were not there to be asked. An auction that ended at 3am while you slept should not cost you the item.
If you miss it
The order is cancelled and the item is offered to the next bidder at their own bid. What that looks like from their side.
Questions people ask
Can I ask for more time?
No. The window is the window, for everybody.
What if my payment fails while the clock is running?
Try again — a failed attempt does not end the window, and you have not lost the item. Common causes.
Does the clock pause if I close the app?
No.
More: you won — what happens now.