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.
Suppose you can swim for 20 minutes, with generous breaks at each end of the pool. You should plan on swimming for 30 minutes, then, so that your actual exercise time (as opposed to rest time) ends up around 20 minutes. To begin, commit yourself to three times a week, 30 minutes per workout.