When cooking or heating up a precooked prime rib it takes around 1 to 2 hours to fully heat the precooked prime rib.
The prime rib is already cooked so it's safe to eat with just heating it up.
But it's best to heat the precooked prime rib up in the oven for 2 hours to allow any potential bacteria in the prime rib to be killed off.
Although it's unlikely there would be any bacteria in a precooked prime rib it's still possible.
When I need to heat up a prime rib that is already cooked I place it in the oven at 350 F degrees for 2 hours which makes it heated up enough and ready to eat.
You could also heat the precooked prime rib on your grill as well for 1 to 2 hours if that is how you like your prime rib.