How far apart do walky talkys work?

im trying to figure out how far the will work with out loosing communication... im going to a friends party ok anyways so she has a big yard and my other friends and i want to use walky talkys...its a long story k so anyways...i think they are pretty good cauality but they are not a new kind but they still work perfectly and they are by a company cobra? ya so if anyone can help me out that would be great...