Here's a quote from
Art and Soul: Notes on Creating by Audrey Flack that I think applies.
"Grunewald spent years of his life on the Iseheim Alterpiece. Matisse's art appears to materialize in a moment. Only the appropriation and major shifts in the usage of time are different. The length of time it takes to make a work does not necessarily make it good. Conversely, the sponaneous and rapid production of a work does not make it good. Supremacy in art lies outside the realm of countable time."