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.
Re: 1 mile css swim times. In BUD/S you swim a 1 mile bay swim to get paired up with a swim buddy of comparable speed. Most of the swims are two miles in the open water. I believe it's 95 minutes pre hell week and 85 minutes post hell week. I believe the third phase time is 70 minutes.