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.
Swim meets can very in lenghth. If it is a slow moving meet it can start early one morning, end late that day, and then go for the whole next day taking 2 days. If it is a quick moving meet it can last for 1 and a half days. The time lenght also depends on the events being offered.