Every year coincident with DefCon in Las Vegas (a hacker/security conference), there is a contest to see just how far a WiFi signal can be transmitted/received to set up a network.

This is run alongside the US 93 highway in eastern Nevada.

Last I heard, the record was 39+ miles, with unmodified 802.11b gear, but with custom-designed antennas up high in the air.

Your antennas, and the coax that feeds them, are the biggest issues in how far your signal will go.