Two years sounds way too clean to me. Hair growth goes in cycles, so some hairs fall out way sooner and others hang on a lot longer. The length of that cycle also changes depending on where the hair is on your body, so a leg hair and a scalp hair are not running the same clock. Dermatologists do say scalp hair grows for a few years on average, but calling it a flat two years for "a single hair" oversimplifies it.

Welcome to the club, I've been pouring concrete for 12 years and just found out last spring (guess I was too busy staring at my slump cone to read a spec book). The 56 day thing is real though, my last pour hit like 15% over design at 90 days with 25% ash. Bad mixes are a whole other animal, had one batch that sat there like peanut butter and we knew we were in trouble.