A rib roast would be better cooked at higher temperatures -- say 325 degrees. At roasting temps like that, I figure a rough guide of 15 to 25 minutes per pound. However, that figure assumes a fairly good size roast -- small roasts sometimes take longer per pound.
Best bet is a probe thermometer in the center. I generally pull a roast beef off in the 130 - 135 degree range. That will give a rare center and medium towards the ends.
In any case, you shouldn't have any problem roasting a rib roast at 325 degrees in an hour to an hour and half. If you are really pressed for time, you can always slice it into rib eye steaks.
I use to eat at a little joint called the Wagon Wheel south of Hattiesburg, MS. They would cook prime rib, leaving it very rare. Then, when you ordered, they would take a nice thick piece and slap it on a wood grill like a steak for a couple of minutes to finish it, serving it with bar-b-q sauce. Wow. Good stuff.
