I think it depends on hot it
is. The recommendation is to let the exhaust gas temperature cool down
to 300 degrees. I don't do that, but I will let it idle if the
temperature is above about 400 degrees. Under normal running conditions,
it will only be about 400-500 degrees.
Of course, you need a pyrometer in
order to tell just how hot it is. The regular engine temperature gauge
doesn't move much at all, despite the differences in exhaust gas
temperature.