A part made from 6082-T6 aluminium or 316L stainless steel can usually start within a few days, because stockholders keep those grades on the shelf (sample note).
Less common grades and sizes may have a minimum order from the mill, or a lead time longer than the machining itself. If the drawing allows an equivalent, say so, and we quote both.
Whatever the grade, the 3.1 certificate is traced to your batch through the route card.