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.
The night before a swim meet eat a dinner that is loaded with complex carbohydrates. Some good meal choices are pasta or chili with cornbread. Carbohydrates help to fuel muscles. Eat a meal 2-3 hours before a swim meet. About an hour before your event, have a small snack.