Our shop receives 24 boxes of Opus a year as an Opus account.6 boxes 4 times a year. Anejos and Hemi maduros we receive 2 times a year, Fathers day and Christmas. Last year our Fathers day shipment (as well as many other B&M's in our region) was lost to the Casa Fuente opening. Some stores who have had Opus accounts much longer than others receive additional or larger shipments of the maduros and anejos. I'm pretty sure the Opus allotment is standard though.
Part of the reason some of you may see Opus at times when there seem to be none is the fact that some shops only display one or two boxes at a time, limiting the number of sticks per person. Doing this allows the supply to be rationed so it appears that a shop has Opus in stock all the time. Another reason could be that some of these retailers dont have the market for Opus, or dont create a market for Opus, such as not introducing new customers to the product or down playing it to people who dont know any better.
Our policy is to display all our Opus and such with no limits on the product. Therefore they usually dont last very long at all.