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.
A lap can be either 25 meters or 50 meters. In the Olympics and major National events they swim 'long course' events which means the pool is 50 meters in length. However, there are still short course swim meets like 'Fina' and the pool is only 25 meters.