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.
According to Nyad's Twitter, she officially spent 52 hours, 54 minutes, and 18.6 seconds swimming. The Florida Strait has been conquered only once, by Australian Susie Maroney, who used a protective cage during a 1997 swim.