Depends on the terrain, frequency, sensitivity of the receiver, noise immunity, type of carrier, quality of information transfer, presence of interfering signals, and in some cases, time of day (did I forget anything?).
Will this be over land, underground, through a tunnel, over/under water, in outer space, etc.?
You would need to know the transmitter antenna gain and its radiation pattern. The field strength meter would need to be of a calibrated type. There would then need to be a (complex I'm sure) mathematical formula to get an approximate power reading. Thus it would be easier to contact the owner of the equipment and ask about it. I am not trying to be funny here but what you ask is a difficult thing. It may be worth telling us why you want to know, and this may also be helpful to us and this will enable us to help you.
