Q: How far can I find open Access Points?

A: The distance you get is very dependent on many factors such as (in order of importance)

(1) Gain and directivity of your antenna

(2) Output power of your Wifi radio

(3) Cable losses between your radio and the antenna

(4) Altitude and clear path of your signals

(5) all of the above, but for the access point you are trying to get into.