First and foremost, the facilities manage shift requests on their own. With that being said, there is not a given time on when your request is going to be accepted or dismissed but you will receive a notification and/or email when the shift has been scheduled or cancelled.
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 that you only get paid for the time you are in the facility working, even if it is a late call.
Please contact the facility if you have any questions regarding a pending shift request or an upcoming scheduled shift.