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.
You can try to swim with your baby step by and step. Usually, it is recommended that swim for 10 minutes at the first swim session, then next time, swim for 20 minutes. Babies under 1 year old should not swim for longer than 30 minutes at a time.