It depends on the cathedral. Cathedrals built in any of the ornate, traditional, stone styles are very labor intensive and costly to build. read more
Even if the money were available all at once it would still take a long time to build and that's if you can find enough skilled stone carvers. Modern styles such as the St Mary's in San Francisco are constructed much more quickly. According to Wikipedia that took about four years. read more
The time it took to build a cathedral in the Middle Ages varied quite a bit since construction was always dependent on things like continuity of funding and support, a lack of engineering ‘issues’ (like towers falling down), and a certain level of peace and prosperity in the city. read more