Depending on where you are located, you probably should have coverage within the same county a site is located. Beyond that, as you mentioned, there are so many variables involved that it would be hard to give you an answer.
The helicopters are like any other user on the system. If a radio with one of the helicopters is affiliated with your site, you will hear that service anywhere they are in the state. For example, I'm located in Allen County (Lima). Lima-Allen County Paramedics use the MARCS system and I can hear their traffic anywhere they go in the state and they go to Cleveland and Columbus frequently. Someone in Cleveland won't hear their traffic until one of their units afficliates with a Cleveland site. Then anyone monitoring their talkgroup will hear all the traffic. Once they leave Cleveland, they won't hear the traffic. The same thing is true with any of the helicopter services.