It really depends on what method you are using for your cast on. If you are using the long tail method, you will need about twice as much yarn pulled off to allow you enough to complete your cast on.
Other methods simply use the yarn as you need it, straight from your ball or skein.http://www.wonderful-things.com/newknit.htm
This site shows about three different cast on methods, and with the illustrations, you can see how much yarn is used to get set up to knit.http://www.anniesattic.com/knitting/content.html?content_id=148
This site shows the knitted cast on, which doesn't require you to figure out how much yarn to pull off, like long-tail.
Me, personally, I like long tail. I always pull off way more yarn than I need. I then cut off whatever is left over after I get my required number of stitches cast on. The excess yarn is never really wasted, since I can use the excess to sew up a seam, hold extra stitches, even stuff some of the small cat toys I make (my kitten doesn't destroy them, so if the toys are knitted tight, she can't get the stuffing out!)