Wireless AP's query?

I know this is an odd request, and I don't know if i'm in the correct
group, but i'm trying to plan out a wireless network - can anyone
answer the following question?

Indoors, in an open space 802.11g what range to you think you'd get on
a roof mounted AP?

20 metres?

Am I making sense?

Found this to be a tough one, no manufacturer seems to want to pin
down a specific range for their APs?

Regards, James