The test period varies depending on the size of the body of water you are measuring along with the size of the leak. If there is a large leak, you will be able to identify it quickly and may not need to go the whole test period. Keep in mind, the longer you test, the more accurate the results will be.
Recommended test times are below:
Average size pool: 12-15 minutes
Spa: 7-10 minutes
Large pond: 20-25 minutes.
Olympic size pool: 30 minutes