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.
Go to bed as early as possible beginning days before your swim meet, especially if you are getting up early. If you go to bed at midnight every night for a week and get 5 hours of sleep every night for a week, getting 10 hours of sleep the night before is not going to help you.