Often times on SCR inlet or outlet duct that I have worked on, the fabricator of the duct presented their Leak Test procedure for review.

There are variables that can affect how long to reach all seams etc. Normally a minimum of 30 minutes was required with NO maximum. I have seen new leaks come up as much as a day later. This was often the result of external stiffeners covering seams and thus limiting the ability for the liquid to get to the area by direct flooding.

Also a tip I used for verifying a "leak" was not from a oil mark or other liquid on the inside is to rub the area with some railroad chalk. If the dark spot grows, it is liquid coming from the other side. Also be aware that repaired areas should have fuel re-applied and wait the same amount of time.

Gerald