Looking back on Lion, DP 1 was released on Feb 24th, DP2 on March 31st, DP3 on May 13th, and DP4 on Jun 6th.
So taking the time differentials, thats something like 25-45 days between releases roughly.
Mountain Lion DP1 came out on Feb 16th. I'll place my bet and say that the next preview wil come out at the end of next week.
Well, there were several updates to the DPs during that time as well. The full schedule for Lion was:
## --- Build # ---- Date ------------ Interval
01 --- 11A390 ---- Feb 24, 2011 -- 4 Mos, 4 Dys (from announcement)
02 --- 11A419 ---- Mar 30, 2011 -- 4 Wks, 6 Dys (from last seed)
02a -- 11A430e -- Apr 15, 2011 -- 2 Wks, 2 Dys (from last seed)
02b -- 11A444d -- Apr 29, 2011 -- 2 Wks, 0 Dys (from last seed)
03 --- 11A459e -- May 13, 2011 -- 2 Wks, 0 Dys (from last seed)
04 --- 11A480b -- Jun 6, 2011 ---- 3 Wks, 3 Dys (from last seed)
04a -- 11A494a -- Jun 15, 2011 -- 1 Wks, 2 Dys (from last seed)
GM --- 11A511* -- Jul 1, 2011 ---- 2 Wks, 2 Dys (from last seed)
The time between the 1st & 2nd was a month, then a new version or update every two weeks (on average).