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.
1 Under the supervision of a currently qualified certified aquatics instructor, BSA or equivalent, participate in four hours of training and preparation for distance swimming (one hour a day maximum).  Swim 1 mile over a measured course that has been approved by the trained instructor who will supervise the swim.