how long does it take to cook a prime rib - 25#

That is rather large. You sure it's 25 pounds? How many ribs?

Wow, that's very large! Do you really mean 25 pounds? Generally, prime rib takes about 15 minutes per pound but it depends on what temperature you use. Many recipes have you start at a high temperature, like 450 F, for 15 minutes, and then reduce the temp to 325 F for the remainder.