He didn't actually complete very much while he was there, though (this was a recurring problem for him which only got worse in later life) -- his contribution over sixteen years was a couple of paintings [including, it must be said, the famous Last Supper, which would rapidly deteriorate after completion] and assistance with various pageants. His one attempt at something lasting and challenging, a giant bronze statue…
Plus, the Last Supper is a huge fresco, not just "a painting", which from concept to execution took several years.