I seem to recall seeing temporary specified as for a maximum period of 3 months somewhere.
C-H mentioned houses: There are still many "pre-fabs" (pre-fabricated buildings) in England which were erected immediately after WWII to help the post-war housing shortage. I know of some people who have been living in such homes for well over 50 years and say they'd never move into anything else.
Back in 1986 I moved into a house which had been erected as a temporary army canteen in 1916. Apparently it was converted to a home sometime in the 1920s. It was finally pulled down after I left for the U.S. in 1995. 79 years for a temporary building - Not bad!
But back to wiring, I see extension cord "temporary" supplies which have become part of the permanent wiring all too often. People seem to string (or daisy-chain) the extensions then build over, panel, or plaster over the cords during remodeling.