Cook time for loins like a Ribeye aren't determined by weight, but rather by thickness of the loin at it's shortest dimension. In this case, it would be it's height. @225 or thereabouts, it will probably take around 4 hours. Put a temp probe in and monitor the cook and adjust cooking temps as needed to hit your target time. I've had times where I turned it up to 250 to finish quicker, and also times that I dropped temp down to <200 to slow the cook down. That said, try to stay under 260 chamber temp.
As to what internal temp to take it to initially, that depends on what "temp" you like your meats, i.e. rare, mid rare, medium, yada, yada, yada. Also, if you want to do a reverse sear on it to get a nice crust, you'd want to pull it about 10 degrees sooner.
I like my Prime Rib to be just between rare and mid rare, so I pull it at 127ish. If I opt for a reverse seer, I'll pull it about 115 and then roll it around on the hot grill to get a crust. Put the probe back in during the seer to monitor internal temp.
A good thing to reference is BearCarver's Prime Rib threads. You can find links to them here.
Scroll down to the "Smoked Beef" section:
Bear likes his PR cooked just a bit more than I do, which accounts for the different IT temps.