When I was learning to call, I was taught to do about 15 times through the dance (maybe 13, maybe 17). I like Victor's suggestion of 8 minutes for the dance, 2 for walkthrough, and 2 for dancers to line up and chat before the walkthrough starts. Including break and waltzes, in a three hour dance, you should easily fit 12 dances in with that in mind. At my local 3-hour dance, we do a 15-minute break, and each waltz lasts 5 minutes (if that). That leaves 155 minutes for a caller to run her dance, and that's plenty of time if you actually run the dances for 8 minutes (you've got 11 minutes to spare using Victor's calculations, which is enough time for unexpected things like longer walkthroughs, a hambo after the break, etc).
In my experience as a dancer, though, callers wildly underestimate how long they are running the dances (and walkthroughs). We rarely get 12 dances in a night at my local venue; usually 10 or 11. This could be because I usually dance in a very long hall, where it is not possible to call dances such that dancers go all the way through the line then halfway back (as Jack suggests) without running dances very, very long. But I also think there's a bias in callers toward thinking they have run a dance shorter than they actually have. I have very rarely thought a dance ended prematurely, but I often find that dances run long and get boring, especially after 20 or more times through (which is frequent at my venue, though when I talk to them, callers generally don't realize that they've run dances through that many times).
My advice is, don't pick a time to run the dances. Pick a number of times through, and do not vary that number significantly based on the size of the hall. Running a dance through 17 times at a bigger hall is fine, but running it through such that every dancer dances with each couple in the set is ridiculous when you're in a hall (like my local hall) that can fit 30 or 40 couples in a line (not exaggerating, I've seen it there). If you decide on a number of times through and actually count, you can stick to that pretty well, but if you decide a running time, it is much more difficult to stay on track of the time and warn the band three times before you want to end, etc.
-Dave