If there was nothing in the agreement about the time frame or refunds, you do not have to return any money UNLESS and only if they were to sue you on the basis that the images were not merely not to their personal satisfication, but were below generally accepted commercial quality (i.e. were not the level expected of a professional photographer or for which people pay). If they did sue and could prove that, they could potentially get all their money back (and return the pictures; they don't get money and photos), or alterately, keep the photos and get some lesser monetary compensation, but would almost certanly need (unless the photos are obviously unacceptable on their face; e.g. as blurry as those "Bigfoot" photos people sometimes claim to take) to hire another photographer to testify at trial about how and why these are unacceptable.