How do I estimate the transmission distance of a 50 watt 2m radio?
Providing that the ground between transmitter and receiver is flat the following formula gives the distance to the horizon:

\$ D =1.2\sqrt{H} \$

where H is the antenna height above the ground in feet and D is the distance to horizon in miles. You then have to double this distance so that both antennas see the horizon. 

So with antenna height of 7 meters (approximately 22 feet) this gives 

\$ 1.2 \sqrt{22} = 1.2 * 4.7 = 5.6\mbox{ miles} \$

Double this to give 11.2 miles.

This assumes that both stations are at the same height above sea level.  If one station is higher than the other, the answer will be greater than 11.2 miles. Providing that there are no obstacles between the two stations (buildings, hills, mountains, etc.) it should be possible to cover this distance with only 1W.