I just found this piece of fabric in my collection. I thought it was an old Hoffman but it was a P&B Naturescape by Jennifer Sampou. Isn't it luscious?
I usually have on hand enough fabrics to choose from for a binding but occasionally I have to go out and buy something special for it.
For a baby quilt 1/2 yard is a safe bet. You will have a little left over for your stash.
Most people cut their binding 2.5" wide but it should really be cut a specific width (math again) for any width you would like it to finish at (maybe a 1/2" binding on a really big quilt which needs 3.25" strips, not 2.5). Anyway, for the people who always cut it 2.5", most quilts take 10 or less strips for binding (not counting king size) so 3/4 of a yard of fabric would work, but I always buy 1 yard in case of a mis-cut and to assure me a small piece for the scrap collection.