How long does it take to write an epic poem?

I should not think of devoting less than twenty years to an epic poem. Ten to collect materials and warm my mind with universal science. I would be a tolerable Mathematician, I would thoroughly know Mechanics, Hydrostatics, Optics, and Astronomy, Botany, Metallurgy, Fossilism, Chemistry, Geology, Anatomy, Medicine—then the mind of man—then the minds of men—in all Travels, Voyages and Histories. So I would spend ten years—the next five to the composition of the poem—and the five last to the correction of it. So I would write haply not unhearing of that divine and nightly whispering Voice, which speaks to mighty minds of predestinated garlands, starry and unwithering.

—Samuel Taylor Coleridge