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.
USA Swimming posts time standards for races measured in short course yards or long course meters. For the Junior National Championships in August 2017, for example, the time standard -- or cut time -- for the 50 freestyle swimming event is 22.89 seconds for the girls for SCY and 26.69 for the girls for LCM; for the boys in the same event, the time standards are 20.59 for the SCY and 24.09 for the LCM.