I usually do things basically by feel, and I also pay particular attention to tune changes and give the band enough time to play the 3rd tune enough times.  I don't have any "rules of thumb", like watching a particular couple go up the line and back, or any specific time length or number of times through, because that gives me too much to keep track of.  

Weather is also an important factor, especially in halls with no temperature controls (like Glen Echo or Baltimore's Lovely Lane).  In wintertime I might run the dances a bit longer to minimize time not dancing so people don't start to get cold.  In the summer, I might run them shorter because with all the sweating and water loss I don't want people to get exhausted. 

The number of dances in a particular evening also is determined by a number of factors.  For evenings where there are a lot of beginners and I have to take some time for walk throughs, I generally can fit in 10 to 11 dances in an evening.  For those where there are more experienced dancers, walk throughs are shorter, or I can do more no walkthrough dances, and I can get up to 12 dances or even more in an evening.  One evening I was in a particularly nice groove with a hall full of good experienced dancers, and I got in 14 dances that evening.  I think the average is about 4 dances per hour (15 minutes per dance, including walk through time and between-dance time all combined with dancing time).

Perry