If your web users are capable of initiating a procedure that might run for five minutes, or even much less than that long, then I suggest that you should be using a proper batch-job processing system to do the work ... and that your web interface should be just that, an interface to that system.   This will give you, in one swell foop, both the ability to control long-running or resource intensive work that has “run away,” but also to regulate the workload that is attempted by the system independently of the number of requests.