If you exclusively pump, how long are your pumping sessions? and Q re 2nd let down.

IBackBevo

Usually, I pump for about 20 minutes or until I notice not much coming out (or nothing come out ) and my breasts feel drained and then pump for another couple of minutes.  This morning, I was messing on the computer while pumping and lost track of time.  I looked down at about the 35 minute mark and noticed that I was getting milk out of both breasts...it was almost like a second let down or something.  Is this unusual?  Anyone else experience this?  Should I start pumping for longer periods?