VHF Radio horizon... (HF is very different):
Under normal conditions (whatever that is), the radio horizon in nautical miles is 4/3 times the square root of the height in feet.
If you are at 10,000 feet, your radio horizon is 133 nm. If the other bloke is at 3600 feet, his horizon is 90nm. Between you, you have a theoretical range of 223nm.
Signal strength drops off with distance, so he'll be faint if he's a long way away.
Atmospheric conditions can do strange things to that - I've contacted amateurs in Northern Sweden, around 1,000 nm away, on VHF with an antenna at 40 feet at my end. That doesn't happen often.
Some VHF ATC stations operate "bandboxed" - covering more than one frequency. Sometimes they retransmit the signal they are hearing - so what you think is someone far away is actually that signal being retransmitted by the local London Info.