That would depend on what equipment and infrastructure you have. For example, hand-portables on a back-to-back basis would cover approximately one mile. However, this is dependent on buildings and indeed terrain. If two people were stood on top of two mountains with a hand-portable each, they could probably speak to each other if they could see each other. On the other hand, if mobile radios are installed in a vehicle or have a base station, much further coverage can be achieved.