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.
The lifeguard test consists of a timed swim, a dive test (see next paragraph), and an interview. The swim test is 500m in the St. Francis Community Center indoor pool that must be completed in 10 minutes or less. 500m is approximately 550 yards, which is 11 laps, or 22 lengths of a standard 25 yard swimming pool.