For males in their 20s:
99th percentile - Less than a week
90th percentile - A few weeks, 10 MPW
75th percentile - Half a year, 20 MPW
50th percentile - 2 years, 25-30 MPW
25th percentile - 5 years of 40+ MPW and intervals
10th percentile - Wouldn't be able to do it regardless of how much training he gets