"Reasonable" depends entirely on terrain, the height of your antenna, and the height of the transmitting antenna. Power is nearly meaningless, except for the local area trunking systems/conventional channels that deliberately use very low power to keep comms local.
Range, rule of thumb: Take the square root of the height of your antenna in feet. Take the same for the height of the transmitting antenna. (See FCC records for this) Add the figures. This is the reliable range in miles. You may often get as much as twice that, but don't count on it.
Mountains/hills/buildings/foliage in between blocking line of sight will degrade signals very very quickly.
If your antenna is up 25 feet, your reliable 'horizon' is 5 miles away. An HT/Mobile is about 5 feet up, so that's 2 miles. 7-14 miles is the expected 'range', with 7-10 being most likely.
The long range of radio comms in public service is almost entirely due to the high altitude repeaters/base station antennas. If your local cops put their repeater on a 1600 foot hill, the 'range' is 40 miles. (plus a bit for receiving antenna height)
Power? Hey, you can talk to the Space Shuttle, 150 miles away, on 5 watts (at both ends)... because it's in line of sight, up there at 150 miles up. Though they may not be able to hear you among all the OTHER stations 'visible' and trying to reach them at the same time.
