As others have said, there is no set duration for a test. It is simply the differential between the pressure supplied vs. the pressure maintained by the cylinder and is accomplished the moment the cylinder is pressurized.
Assuming you remove one plug at a time to test, 10% leak-down would be considered acceptable. 20% or higher indicates problems either in ring seal or valve contact.
Following a "dry" leak-down test, you can pour a small quantity of oil down the cylinder and retest "wet". The oil will improve the sealing of the rings. If the leak-down % improves, the rings are suspect.
Conversely, no improvement can indicate leakage past the valves. This is usually accompanied by the sound or air leakage within the intake and/or exhaust tract.
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Triple Black 1971 GTO
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