How long does it take for arteries to become blocked? 20, 40 years?

Varies. Arteriosclerosis or hardening of the arteries tends to run in the family dna, and some are prone to develop blockage early in the teen years, especially when associated with juvenile diabetes. The blockage starts with a fatty streak in the wall of arteries, and this is seen in teen years/early twenty, and progresses so that blockage is seen in every decade thereafter.