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.
A typical swim session consists of a warm-up, main workout set and a cool-down. Workout sets are often written out something like this: 4 x 100 on the 2:00. That means you have two minutes to swim 100 yards or meters before starting the next repetition in the set, for a total of four times through. Your rest is included in the two-minute interval (so the faster you go, the longer the rest period).