Fourteen days from purchase
Stocked items can be returned within 14 days of purchase for a refund, credit or exchange. The returns page says to call and arrange it. The phone number on the site is +44 (0)208 386 6960.
That window is for stocked items. It is counted from purchase. An item that was made or printed for the order is a different case, and the returns page is the place to read it before sending anything back.
A defect found later
A defect that was not obvious on delivery is covered in the terms. Those terms ask for notice within 14 days of discovering it, and no later than 30 days from delivery.
The return does not change the dispatch rule on the next order. Orders placed by 3pm are still dispatched the same day.