What is the longest period of construction for a modern building?
We don't know, and it may depend on how recent a time is encompassed by the word modern. We'd like to submit the Sydney Opera House, Sydney, Australia as a candidate. It was under construction for 14 years. Another possible contender might be the Baha'i Temple, in Wilmette, Illinois, on which construction was suspended for a long period of time. The cornerstone was laid in 1912. Application of the exterior began in 1931 and continued for 10 ½ years.