Answer:
300 years or half a century
Across 217 church and abbey projects in England, construction took an average of 250–300 years. And St. John the Divine is not alone among the ranks of unfinished cathedrals. Perhaps most famously, Gaudi's Sagrada Familia has been under construction since 1882.Completion of a new cathedral usually took at least half a century, and many took much longer, involving generations of architects and workers. Since construction could take so long and was so expensive, portions of many cathedrals were built in several successive styles, and many were left unfinished.
