I have never seen in writing or heard of a specific time limit the pilot has on the runway when cleared for a S&G. Logic would dictate when I issue that clearance to a pilot, it is the pilot's discretion how long they stay in the "stopped" position.
That being said, if it ever mattered to me there were numerous ways to work with the pilot(s).
Simply asking the pilot how much of a delay they were going to take before rolling was the easiest.
If I needed something, I stated it. IE after getting the clearance read back "no more than 5 second delay on the runway," or something like that.
Heck, I've even given a pilot requesting a T&G a S&G instead for sequence purposes (that would come into play after he was airborn again). After getting the S&G clearance readback, I threw in a "after stopping, count to ten, then go - the delay is for sequence"
Worst case scenario, if I couldn't allow the delay on the runway, or the pilot was having trouble complying with any time restriction I'd just "unable" the S&G. Even when, at times, they would get cute and come right back and ask for the option, I'd clear them "all options except S&G."
Bottom line, I've never seen it in a reg. A complaint about a delay being too long on the runway should only happen once - as there are many ways to work with the pilot to prevent it from happening again.