Right now, OCCT is reliable. No known bug will detect an error if there isn't. So if OCCT detects an error : something is wrong.

However, generating errors is random. There are cases where the computer is unstable and OCCT won't detect it. Usually, the longer the test, the higher the chance of detecting an error. For alot of people, 30 minutes-long tests are enough. You can jump to 2 hours or more if you want to double-check. Some people are known to test their computer for 24h !