Can I Guess How Long They've Studied Korean?

A couple of months ago I had a fan meetup in Seoul, and I tried to guess how long several of my subscribers had been learning Korean for. Some of them had just recently started, while others had been studying for years. So I did my guess to estimate exactly when they started learning, and absolutely failed (at most of them, but not all). Can you guess how long they've been learning Korean better than I did?