Long Versus Short Stretching
When it comes to how long you hold a stretch, there apparently is more flexibility (so to speak) than you might have thought.
A recent study compared the effects of 30-second hamstring stretches to 10-second hamstring stretches. Twice a day, study subjects stretched each leg for a total of one minute: two 30-second stretches on one leg and six 10-second stretches on the other. At the end of 6 weeks, there was no real difference; the subjects had increased their range-of-motion about the same amount in each leg.
Bottom line: it's the total stretching time daily -- and not how long you hold each stretch -- that matters.