They don't publish that because there are so many variables. Installed pulley ratio, engine RPM, cold plate size, box size, insulation etc... I can take our Sea Frost plate down to -18F in about 35-40 minutes at fast idle. Or about 30 minutes at cruise RPM.. Every plate and install will be different.At any rate- Grunert and Sea Frost appear to post specs except for average run time. Likely due to variations in box size and insulation as well as climate issues.
Did some more digging in forums and the short answer is about one hour every 12 hours in the tropics and less in cooler climes. Initial cool down will take longer. A rough average rule of thumb.
The best thing to do is install a temp sensor directly to the plate so you don't over run the engine as that just becomes wasteful... With longer runs our plate will get colder than -18F but we have found -10F to -18F gets us about 12+ hours... Our Sea Frost shore assist was also custom converted by Cleave to run on DC (Danfoss BD 80) instead of on AC so we have both DC and engine driven all in the same holding plate..