http://www.terrypepper.com/Lights/lists/visibility.htm

http://www.pajack.com/stories/pitts/viewdistance.html - thought this was slightly interesting even though its water&lighthouses

gives you are the right idea!

because its will be line of sight communication, distance, terrain , trees buildings are all factors.

or you could buy like 4 of these and kind of set up your own repeater at not too much of an expense.

http://www.aliexpress.com/item/2-4GHz-25dbi-WiFi-Antenna-For-Wireless-Router-Outdoor-Yagi-Antenna-free-shipping/727257342.html?af=ppc&isdl=y&src=Google&albch=Google&albcp=Search_Search-ALL-PLA&albkw=None_US-Shopping-Search-Product-AliExpress_none_none&albag=home_none_Search-ALL-PLA_1013999999870&albmt=broad&albst=search-text&albom=OTHERS_None_20130807_Aliexpress&creative=31447451305&ptsid=1013999999870&gclid=CNG98ZnvnLwCFY07Mgodnz8A_A

again it all really depends on the distance from you.