While doing a coolant pressure test what is the normal time to come back and check for leaks - assuming you don't see any in the first 5 minutes or so.

And, how long should it hold that pressure, it it reasonable to be able to hold it for 3-4 hours?

 

On a cold pressure test, it should pretty much hold until you release it. It will drop slowly but it should hold over night

 

I have held overnight as well.

 

If you're not losing any coolant I'd call it good at 3-4 hours. If you're losing coolant and cant find the source it's time to add UV dye, go for drive to complete a few heat cycles and then retest after dark with a nice UV flashlight. Pull spark plugs if you can't find anything external.


I don't even use a good UV flashlight, I use a cheap one meant to find where animals have peed on the carpet.