If there will be some sort of ceremony (a christening, a naming/bris, etc.) within a few weeks, that would be a usual time to bring a gift. Otherwise, just whenever you visit or are able to drop it in the mail.
EDIT: Baby blankets are cute, but not all that useful. Medical recommendations are that baby NOT have blankets in the crib, so unless the parents opt to swaddle (and the blanket is the right size and shape for that), it will get minimal use.