Totally depends on the size of the roast (how many ribs and the weight) and the cook temp.
I don't use salt on a Prime, bring it up to room temperature, and slather the cut ends with real butter. I also remove the meat from the bone about 90% to the spine end and then use butcher twine to tie it together between the bones. If you've never done this your PB will have a much harder time being the perfect pink you are looking for.
I made a six bone with the above method and seared the outside at 425 for 20-25 mins. Dropped the temp to 325 and added butter to the fat drippings for basting ever 20-30 mins. Took it out to rest at about 125 because the residual heat on that size brings it up to the 140-145 that I want.