The answer depends upon the piping system, its condition, layout and configuration, pipe material, and many other factors.  Generally, a full day of investigation during which 50 or more examples of pipe from one single piping system can be addressed will provide a very thorough assessment of piping condition for one piping system.  Another important consideration is the need to prove an issue beyond debate, and to produce a report showing clear direction to follow.  Wherever dispute exists regarding the condition of a piping system, and especially where litigation is even a remote possibility, additional testing should be planned in order to eliminate all possible debate to the results by all parties involved.

In a past investigation in San Francisco, one day of planned testing at a 32 story office building was insufficient to test every floor of a package unit based condenser water system.  We provided building management the suggestion to choose even or odd floors in order to cover the full length of the property in the one day of allotted time, which we did.  A report was presented two weeks later on all even numbered floors showing every single 3 in. threaded take off connection from both risers in severely deteriorated condition due to high galvanic activity caused by the brass valve.   Our recommendation was to replace every single take-off from the main risers to the isolation valves, on all 32 floors, with dielectric fittings. 

Two months later we were asked to return to San Francisco from New Jersey in order to test all odd numbered floor connections.  Although we documented beyond any doubt that every single example of 3 in. run-out pipe on the even floors was near the point of failure, upper level managers had interpreted our report differently, and had reasoned that the odd floor piping was fine.  Seeing no issues for the odd floors in the absence of hard proof, a critically important pipe replacement project was cancelled.  Four weeks later, we returned to San Francisco to find every single 3 in. take-off section of pipe severely corroded and approaching failure, as one would reasonably assume. 

Where a corrosion problem and pipe weakness is identified, additional time may be required in order to better define the extent of the weakness throughout the system, or to prioritize the greatest threat locations for more immediate corrective action.  In general, we recommend testing an absolute minimum of 25 different locations for any specific piping system in order to produce a reliable piping assessment.