A quilt can be used for tummy time on the floor, so I think they can safely be used from day one. For use in a crib as a covering, I would probably err on the side of caution and wait until the baby can lift his or her head and/or roll over.
However, my son would only sleep on his tummy, and the "back to sleep" recommendations which I do firmly believe in, made him miserable for the first few months - so we did keep covers and bumpers out of his crib for many months since he would not sleep on his back.
Cheers, K