There are too many variables to give you any kind of an idea.

Which antennas are you using?

How high are they?

Where they located, relative to the rooftop (i.e., the area close to the building is likely to be "shadowed" by the edge of the building ... think of it as if it were light)

How much / what kind of cabling, how many connectors, average humidity, trees? ...

And even if you answer all those questions, we still can't give you a number, because the other half of the equation is the client, the radios the client is using, the antennas the client is using, what is the acceptable throughput for the application, how sensitive is it to interference, what interference is in the neighborhood ...

There is no rule of thumb, there is no way (short of some very expensive software models) to predict the coverage ... use a dartboard with numbers from 6-50 (meters), it'll be at least as accurate.

You need to do a survey, or have a survey done. That's the only way.

Before you do a survey, you need to know what the application needs are, minimum throughput, how many users (max) for any given zone within the area, the protocol mix (802.11a, g, b, mixed?) ... and lots more.

Call someone, have a survey done. Otherwise, you're in for lots wasted money and days-to-weeks of serious frustration and complaining users. Reputation is very hard to earn back once you've lost it.

Good Luck

Scott