To see if my lengths of yarn are long enough, I use the finger tip to elbow measuring method :) I hold the yarn between my thumb and index finger, pull it to my elbow and repeat 11 more times. This typically ensures I have enough yarn to complete two rounds of a granny square.
Somehow my first post was overwritten. I like to think it was the blog site, but it was my own user error. I have been away from this site for too long, and it shows!