Facilities have full control over managing shift requests. There is not a set time when to expect your request to be accepted or dismissed. You will receive a notification and/or email when the shift has been scheduled or canceled.
You are welcome to remove a pending request if it no longer fits into your schedule. If a facility accepts your request after the shift time has started and you did not remove your request, you are still expected to work that shift and report the hours correctly.
Note: You will only get paid for the time you are in the facility working, even if it is a late call.