But surely "within reason" includes some sort of timeout?
On a regular day the controller could see this, and ask what is going on. At a small GA airport with training ops the controller is likely used to people pulling out onto the runway and taking their god damn time. The FAA actually advises wait times of as long as 2 minutes for wake turbulence, and that’s something I’ve done. Just sit there on the end of a runway.
Things get a little different, in practice, at busy commercial airports, but the rules are the same.
More than once I remember a 747, 777 or A340 taking off and getting the call "cleared for immediate departure". The pilot I was with routinely would say something like "I'm not even taxing onto the runway for 2 minutes, then we can talk".
So if a large plane has just used a runway, small planes will wait before taking off, whether that’s on the runway or off the side on a taxiway is up to the pilot and the tower controller.