I say at least a yard of whatever you like. Maybe two if the deals are really good or there aren't many different flannels that you feel you have to have.
Like jodiemarie said remember to prewash your flannel. My first quilt was made out of flannel and luckily all the quilting books I read told me to prewash the fabrics. I'm sure glad I did because the yardages of flannel I had washed and dried shrunk about two inches.