The Spire was officially opened in 698. Construction work began officially in 695, but multiple sources cite preliminary work being done starting from 693, and some mention measurements and soil tests being conducted as early as 687. I have found reports of even earlier works, back a whole fifteen years more, but it's near impossible to tell if any of it was really connected to the Spire. There may have been a previous project that was later abandoned and never publicly announced.
By all means it feels like they were rushing construction. Even just comparing the official records to those of any other building of similar size, they were overstaffing the construction site. There is no official data on work hours, but pictures from the time period seem to come in every kind. The ones taken in the morning are as full of workers as the ones taken during the evening, and I've even found a couple taken at night.
They took too long. With so many workers and that kind of schedule the building should have been finished in half the time. There are no reports of extraordinary weather events during the construction period and no reports of malfunctions in Canterlot's weather dome either. They were working non-stop in ideal conditions and the size of the construction materials doesn't justify the extra time. What took them so long? And why were they rushing construction when they could have had the opening coincide with the 700th anniversary of the Reconciliation?