Look around at patterns and get an idea of what you would like to make, then see how much that takes. It's surprising how much yarn it takes to make, say, an afghan or a big sweater. If you go to Red Heart's site, you can see various projects made with a variety of their yarns:http://www.coatsandclark.com/
Maybe that'll help you decide.
I found a skein of cream-colored Plush at Goodwill and have maybe half a skein left over from all the Mobius scarves I made last winter. They're going to be a shawl, I think, but I haven't found just the right pattern yet.