For the last two years, in addition to directing Total Immersion technique workshops, I've also coached the sprint group at the U.S. Military Academy at West Point. I always find myself wondering what they'll have left when the gun goes off for real. In school and USS swimming, I see coaches with their watches out timing sprint after sprint before the meet ever starts. The time to show everyone how fast you are is when the racing starts in earnest. Your warm-up yardage depends on several factors.
Distance races require more warm-up than sprints.
Find a warm-up that works, and stick with it. At West Point, we work out a routine for each swimmer over the course of the season and they'll do it as their practice warm-up during the last week of taper. Those racing longer distances should do some timed pace work to familiarize themselves with how their race will feel, but it should be done as easily as possible and always in something of a buildup fashion, i.e., finish each timed-pace swim faster than you started it to simulate the ideal progressive pace for racing. Whenever possible, use the warm-up pool before and after each race. Prior to races, raise your heart rate to about 140 BPM before the start.