For me, it really depends on the size and energy level of the dance.  For a larger dance with lots of energy, I will run around 15 min per slot...so 3-4 minutes to get a new partner, 3-4 min to walk thru and 8 or so minutes to dance (around 16 times through the tune).  Square and mixers run shorter frequently, doing a square and a mixer means that I get an extra dance in overall.  

Smaller dances, or dances where some of the dancers have problems with stamena,  I tend to run shorter -- frequently more like 10 minute slots.  Have gotten 10-12 dances in a 2.5 hr (including break) dance.  Have a friend who called 14 dances in a 2 hour dance because the band was an old time band and was just playing one tune per dance, the same way every time through, so she just ran all the dances much shorter.  

My goal is generally to get people down the hall and partway back during the dance for a single progression dance.  YMMV