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 there exists some controversy over how long a stretch should be held. Many researchers recommend 30-60 seconds. For the hamstrings, research suggests that 15 seconds may be sufficient, but it is not yet known whether 15 seconds is sufficient for any other muscle group.
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.