I was just speaking on this subject the other day. Warmups at meets are quite variable, depending on the age of the swimmer, the degree of rest, and the events to be swum. During the season, the warmups are typically longer than at Championship meets. Be sure to get the heart rate to max or near max in warm up, regardless. That is a common mistake. We also like to do dynamic stretches as part of the warmup before getting into the water. Typically the warmup ranges from 1000 meters to 3,000 meters, depending on the above.