Opus comes out quarterly, but I have heard that they may be reducing the frequency. At least it has been that way for me for the last year, I could count on being called March, June, Sep, and Dec 15th or close to it.
As for the boxes sitting on the shelf, well, it is the same near me but the shop marks up the boxes 40% so no one wants to buy them.....
Getting full boxes, at decent prices, is all about building relationships with B&M's. Most will not sell a box anywhere close to msrp to someone they do not know.