This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
One thing many people seem to disagree about is how long to hold a passive
stretch in its position. Various sources seem to suggest that they should
be held for as little as 10 seconds to as long as a full minute (or even
several minutes). The truth is that no one really seems to know for sure.
According to `HFLTA':
Some controversy surrounds how long a stretch should be held. Some
researchers say 30-60 seconds; more recent research on the hamstrings
indicates that 15 seconds may be sufficient. Whether the 15 seconds
that may be sufficient for the hamstrings is also sufficient for other
muscle groups is unclear.
A good common ground seems to be about 20 seconds. Children, and people
whose bones are still growing, do not need to hold a passive stretch this
long (and, in fact, Kurz strongly discourages it). Holding the stretch for
about 7-10 seconds should be sufficient for this younger group of people.
A number of people like to count (either out loud or to themselves) while
they stretch. While counting during a stretch is not, by itself,
particularly important ... what is important is the setting of a definite
goal for each stretching exercise performed. Counting during a stretch
helps many people achieve this goal.
Many sources also suggest that passive stretches should be performed in
sets of 2-5 repetitions with a 15-30 second rest in between each stretch.
 
Continue to: