As a sporadic weaver I am always unsure how long my fringes should be. I know that for items like baby blankets, fringe should be short, so a baby's fingers don't get caught in them. But when it comes to scarves and shawls, should they be a prescribed length, personal choice, or is there some general rule of thumb to go by?
I don't think there is a rule of thumb about fringe length. So many factors are involved— the yarns, the proportions of the piece itself, the thickness of the fringe (both yarn thickness and/or the thickness made by the number of threads you include in each fringe for a twisted fringe). I think these are generally personal choices.
What I like to do (for scarves and shawls) is to allow from ten to twelve inches unwoven warp at the beginning and end of each piece (usually secured by hemstitching or by a few picks in a contrasting color that I remove if I'm sure I'm going to make a twisted fringe). When I take the piece from the loom, I decide if I like the fringe as is (if I've hemstitched) or want to twist it. Then, I work a few twisted fringes, seeing how many threads look best in each fringe. Sometimes I like the fringe to be relatively fat (especially with wool fringe). After I decide what I like, I twist a few fringes (removing the securing picks as I twist them) and tie an overhand knot in the end (this fringe is usually about eight inches long). When I see how this length of fringe looks in relation to the scarf or shawl, I decide how long I want the finished fringe to be, retie the knot at that length, and twist and knot the rest of the fringe that way.