You don’t mention whether you’re using B or G, so I’ll assume B since it’s longer range. With that kind of gear, in a quiet rural setting, I’d bet on 10 miles or more. Swap that CM9 for an SR2 or similar card and you might push out to 15, figure 10 with a solid 11Mbit link and falling off to 1Mbit at the far range.
If you’re in a dirty RF area, cut those ranges in half, or more.
A lot of this depends on where that omni is also. If it’s up high where it can hear every signal for 30 miles in every direction, all that hash keeps it from hearing a gap for it’s own transmits.
If your omni is already mounted in its final location and you only hear a couple of residential AP’s on a scan, yep, that’s a quiet area. I’ve seen one up high on a hilltop come back with several screens of other AP’s, the site was pretty much unusable.