How often do you need to check torques on bolted flanges?

Question

Due to the fact that pipe flanges rely on bolts, and bolts rely on friction, it seems obvious to me, as a manufacturer of piping, to recommend that bolt torques need to be checked every so often, to ensure piping doesn't leak. I've got a few questions about this, but here is the first one:

How often do you normally check torques on bolts at a plant? In other words, what is a good industry standard preventative maintenance schedule when it comes to threaded fasteners that are loaded below yield? I would prefer that answers provide a reputable source for reference.

Answer 1

There is no answer because there are too many variables, amount of vibration and thread geometry being just two of them. A joint could last for 10,000 years or work loose tomorrow. There is no way to know.