Originally Posted by goodeats21

I would shoot for somewhere around 1.5 to 2 hours.

If you miss the connection, doubt there would be any good way to get to KEF without an overnight. So would not really push for tightest connection possible.
KEF will depart from Terminal C.

I have a lot of experience with CVG <-> EWR. Mostly good experiences, any delays have been sub-1 hour.
GoJet has (almost) exclusive use of Terminal A at EWR. So if GoJet is the operator, plan on Terminal A.
The other UAX operators will use Terminal C.
Allow about 15 minutes +/- for the transfer from A to C via airside bus.

LAX->EWR will be the PS route, so great ride in First if you are going that way.