This means that the battery is discharged down to 10.5 volts over a 20 hour period while the total actual amp-hours that it supplies are measured. Sometimes ratings at the 6 hour rate and 100 hour rate are also given for comparison and for different applications.
Pool meets are usually no more than 4 hours, but beach meets last about 7 hours. It depends how many events are in the meet. And then if it is a really big meet (like a tournament kind of) it can last days. Hope that helps.
The best swimmers among female Ironman triathletes complete the swimming portion of the event in 50 minutes to an hour. At the World Championship, the cutoff time for all competitors is two hours, 20 minutes. Any participant who fails to complete the 2.2-mile swim within that time is eliminated from the competition.