There is no standard length, a hank could be 80yds or it could be 400yds or anywhere in between. You could weigh it but it still wouldn't really help you know how much you have in length. What I did once, an idea I saw on a website that I don't remember right now, is take a board of wood and nail two nails, 1yd apart. I wound my yarn (it was from a recycled sweater, no idea of the yardage) around it and counted how many times it went around (one full time around is 2yds), then I had a pretty good estimate of my yardage. To protect the yarn itself, I put drinking straws, cut to the right length, around the nails before hammering them in so there was no chance of snagging the yarn.