EDIT: Ok, in answer to the edited question, I'm completely redoing the answer. I did leave my old answer below if anyone wants to read it.
The distance two towns can be apart and still be under one regime is infinite. But the distance between two towns that a single governor/monarch can handle (i.e. no act in, besides maybe a simple mayor to handle civil disputes), would be about a week ride. Because the monarch would have to go there to see exactly how things are doing. A Garrison of solider would be under his command, not the "mayors" so he would need to be able to give them commands.
Social problems could reduce the distance. If there was a high chance of an uprising, the monarch would need to be closer to be able to keep a better eye on things.
That would definitely depend on what technology the civilization had. With even current day human technology, world-wide domination could be considered possible (the English did it with even less sophisticated technology). Instant communication, and even better, instant travel would be huge in deciding how much distance apart two areas could be and still be ruled by one person.
As suggested, good, local authorities would also be needed for any sort of distance. A ruler can not be in any place all the time, even with the above technologies. But a good local ruler can keep an eye on things and make basic decisions without his leader.
The leader would also need to keep in touch with the people. If you don't know what your people want/need you won't be able to keep them happy/rule them. Or the leader could be a dictator, but either way the leader would need to know where problems were. This is complicated by larger distances, so that can put a limiting factor.
So it is hard to give a number on how far away two areas can be to be ruled by the same person, a specific set of technology/circumstances would need to be given for that.