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.
By contrast, the 2017-2020 National Age Group Motivational Times for 10-year-old girls in the AAA group for the 50 freestyle swimming race are 32.79 for the LCM and 28.89 for the SCY; for 10-year-old AAA boys, the standards for the same race are 31.89 for the LCM and 31.59 for the SCY.