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.
There are a few easy ways to figure this out. First of all, most sprint triathlons are of a similar length, but the distances can vary. For a novice tri-athlete, it will take about 2:00 minutes per 100 meters for the swim, or 8 minutes total, about an hour for the bike (avg 12mph) and 30 minutes or so for the run.