That is why you have constant blockages on the narrow steets and endless construction projects.

(Often you can "solve" low flow of a hot water faucet by changing any/all elbows you can access from the water heater to that faucet.)So, the short answer to your question is...when repair costs due to plumbing leaks and/or blockages start rising notably, or you are observing several signs of hidden leaks.Once you are past the 25 -30 year mark, you are bound to have more leaks, stoppages, and low flow (as well as prior, poor, repairs).

They are so hard on washers and dryers that you're constantly replacing them or fixing blockages or cleaning up overflows that create water damage.

Anyway, I called a plumber, and they got the backup cleaned, found the line, popped an "emergency" diagnosis hole in it so they could scope and found the 4" cast iron line rusted through in the bottom which was causing the periodic blockages. 

The sewer was backing up and I paid way too much money trying to get others to clear the blockage.

It was either a blockage caused by the tenant, or a main line blockage that may be a common line with some shared responsibility depending on the property and its rules.

This is also another reason I always do a move in inspection WITH the Tenant before handing over keys; and, why I take the time, during the signing of documents, to explain the repair request process and specific preventative steps they can take for items such as disposal blockages and other minor, routine, issues.

He was dead set against replacing it as it was up high on the hub where nothing was likely to get caught and cause a blockage.

Drain stoppages need to be documented by the plumber as to how far out the blockage was, what did they pull back, if anything.

This is a current blockage in the goal setting as I don't know how much knowledge transfers from single-family to multifamily investing.