Testing software is like cooking popcorn is a microwave. With popcorn you set the timer for five minutes but it's time to stop when popping slows to 2-3 seconds between pops. With software you can estimate how long testing is going to take based on your past experience, and the amount of code to test, but the only way to know for sure when the software's done is watch for when the interval between finding bugs goes up beyond some limit you set.