The Temple Expiatori de la Sagrada Família, called Sagrada Familia in everyday speech and "still not completed" in the city planning office, is a Roman Catholic church that's been under construction since 1882. Construction is expected to finish in 2026 at the earliest.
Gaudi, its designer, intended it to be the "last great sanctuary of Christendom" when he designed it. A brief account of the time taken[a] is on Wikipedia, and a more detailed history[b] is on the church's own website.
While the project management certainly can be faulted for failing to complete the church in reasonable time, and Gaudi for continuously revising his drawings in a seemingly crazy pursuit of perfection, it should be noted that cathedrals have always taken very long to build. For examples, the Stephansdom[d] of Vienna is still unfinished 860 years after construction started, and the Cologne Cathedral[e] took 632 years (1248 - 1880) to finish. Even in a country frequently blamed for having a short attention span, the United States, the Washington National Cathedral[f] took 83 years (1907 - 1990). If Sagrada Familia finishes in 2026 it will have been under construction for 144 years.
Finally, one should not underestimate the virtue inherent in creating a building that glorifies God. The construction can in itself be seen as worship, even if no proper mass is held in the unfinished structure.