Board Thread:Theories and Speculation/@comment-28323245-20160902000709/@comment-734582-20160902085128

Considering the clones and time involved, the equivalent of some 3 years of non-stop training.

Since other elements seem even more difficult if they're not your affinity and Naruto already sucked at using Wind Release beyond this, my estimate would be a decade per element.

With a few hundred clones that's about a week.