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.
On my Master's swim team, the slow lane (where I swim) does about 1000 yds per half hour if we're swimming freestyle (fewer yds for other strokes per half hour). The fastest lane does 1000 yds in 20 minutes or less. So my lane aims for 4,000 yds in 2 hrs, while the fast lane aims for 5,000+.