Usually a couple of hours
Hi
Is your answer based on your experience on websites? If so, I guess that could be the case, if websites are not correctly cancelling NRS reservations when a browser session is ended, or a basket is emptied, for instance.
Ticket Issuing Systems that are accredited by RSP (i.e. all of them) are supposed to return the availability to NRS immediately a user does an action. It's only two hours if you open a browser window with an Advance fare on it and leave it open without doing anything.
NRS has got its faults, but instant availability isn't generally one of them.