I dont know the length. I grab the tail, pull it out 4 times (4 "sets" of thread from the skein), and clip one thread at a time (I use loop start). I then wrap the remaining (still attached to the skein) around the label to ensure it doesnt fall off.
About 90cm (length from shoulder to fingers x 2) because I do a loop start. With 45cm (half) I can pull the thread nice through the aida in one time :)
I think every one has their own method - just the way you like!!