It depends on the pattern and the fabric I am using. If I am using a higher count fabric like 28 count, I only cut 12" lengths. If I am working on 18 count, I will go to 18". The reason for this is that the longer the thread, the more times it goes in and out of the fabric, and eventually it will lose its shine. With a high count fabric like 28 count the stitches are much smaller so even 12" is enough to go in and out many many times. That way it is also much less wasteful when I need to do just a few stitches in a color.
I like to use a pretty long strand. I just hate cutting a new piece especially when I really get going on stitching. I feel like stopping and starting with a new strand takes me out of my flow. Talking like a good 2 arms lengths if Im doing 2 threads because I use the loop method for two strands. If it is one thread, Ill still cut it pretty long, 30 inches or more, Id say.
I use a loop start so I double the length to give me two strands, I finish up with about 50 cm.
Depends on the thread used, metallic threads are more of a problem - you need shorter lengths and a useful tip is to knot the needle end so that the threads dont keep slipping.
One useful tip to stop knotting is to wet the thread very slightly with your mouth!!!! It makes the fibres stay flat.
Twisting threads are a problem - you can railway line them if you are using two strands by placing the needle between the two strands before stabbing the fabric.
I usually use about 19 inches, unless Im using metallic thread, and then its about 12 inches. Because I pre-cut my floss at the beginning of a project (except the metallics which I try to handle as little as possible), I like them to be all the same length in case I am working on a pattern with blended colors (Teresa Wentzler, I am talking about YOU!). I find that longer lengths reduce the shine, and they can even fray slightly, which is unsightly.
I pull from the skein about 5 bends (about 36in or about 91cm) then use with thread heaven no knots! I dont like to have a lot of buried ends making the back look messy. :)
chekccard