That is a Reg B allowance. Since your loan request is subject to HMDA reporting you have to report it as approved but not accepted, you cannot report it as a withdrawn application.
As Lisa said the time you hold an approved application is your call. I would agree that 30 to 45 days would be a sufficient time in most cases, of course if a date was specified in a commitment or pre-approval/qualification letter you have to abide by that date.
The opinions expressed are mine and they are not to be taken as legal advice.