How Much Yarn

I am going to make a twin bed "blanket" same size as a comforter & have measured 60" x 85" as my desired size. I have NO idea how much yarn I'll need - it's a shell stitch - could you please help with a "guess-timate"?? It's from a pattern that gives stitch instructions but just says to make whatever size you want - doesn't help me a bit!! Thanks in advance!! Barbara