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.
If you want to swim for 30 minutes, divide 30 minutes by your average lap time to determine how many laps you need to swim. For example, if you swim a lap in 45 seconds, you need to swim about 67 laps to complete a 30-minute workout.