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.
At the end of the swim, record your total time. Then divide that total time by 10 (a 500 yd swim is made up of 10 x 50s) and the result will be your 50 yd swim pace. Therefore, if you swam your 500 freestyle in 7 minutes and 30 seconds, your aerobic 50 yd swim pace would be .45 seconds (7.5 minutes divided by 10).