What pattern are you using for the scarf? It should tell you how many yards you'll need for a given length of scarf. If you're just knitting without a pattern (casting on 20sts so it's 20sts wide), measure out 5yds or so of yarn and mark with a pin. Knit to the mark and see how long your work is. A little math'll give you an idea of how long a scarf one ball of thick and quick will make.
Here's a pattern
that uses 2 balls T&Q for a scarf.