What is the usual range of Approach and En Route ATC VHF radios?
VHF radios are line-of-sight, and typically the range is limited not by the transmit power but by terrain and the aircraft's horizon. PPL students are taught a rule of thumb: the range in nautical miles is the square root of 1.5 times the aircraft's altitude in feet. This makes your range roughly 40 nm at 1000 ft, 90 nm at 5000 ft, and 120 nm at 10000 ft.

En-route stations intended to be used over a wide area will typically use several transmitters on high ground many miles apart, to ensure reception over their whole operating area. Even so, low-level flying can put you into a blind spot between transmitters.