I'd say a minimum of 28 days. If I were doing it... I'd probably go with 35. Make sure you're getting a huge roast- you're going to lose a lot to moisture loss and to trimming. And, also, make sure it's well marbled piece of meat. These days, prime is no guarantee of marbling. All the dry aging in the world will do nothing for a relatively lean steak.
Anyone have Steingarten's books handy? In one of them, he talks in depth about how the prestigious steakhouses (used to) do it. From what I understand, dry aging ain't what it used to be. The duration keeps getting trimmed and trimmed due to the potential profits involved and less than savvy customers.
Whole Foods is supposed to be dry aged, but every time I've spent the paycheck and bought a steak it was still very watery and tough. If memory serves me correctly, they say they age their steaks 21 to 28 days. I worked there for a bit, and can tell you, for certain, no steak ever makes it to 28 days. As long as the demand is constant (and the demand is always constant) they turn over these steaks as quickly as they can get away with. Just between you and me... I don't think many of the steaks even made it to 21 days. From the butcher- sorry, meat team leader's perspective, every day past the barest minimum those steaks stayed in the case, he/she was losing money from water evaporation. If you've got people knocking down your door for aged steaks and you've got 14 day old steaks in the drying unit... are you really going to say, sorry, come back in a week?
Anyway, I digress. Get your hands on Steingarten. He's got the historical perspective on aging, which, imo is the authority (not Steingarten, but the old time steakhouses).