If you're doing stretches to cool down or plank poses for strength training, how long you hold each move is crucial. I like to stay in each stretch for 30 seconds, while I do the plank for different amounts of time depending on how much of a workout I want. But I often find that my perception of 30 seconds is skewed from reality!
Here's a fun trick to try: If you have an iPod with a display, use the song counter to time your moves. Position the iPod on your mat in your line of vision, and watch the numbers tick away. That way, you won't lose count, and you can be sure your 30 seconds is a real 30 seconds.