A lot of folks talk about stopping when barrel surface temps approach instantaneous scalding temps (>140F), but something to consider, steel isn't changing until we're more than 10x that temp. Great safety measure to not get a barrel so hot that you can't safely touch it, but in terms of barrel life, the flash temperature (and resulting heat from the internal strain of the pressure pulse) during the shot is causing ridiculously more change to the steel inside the bore than anything we'd be able to influence by shooting faster or slower with a bolt action, or anything we'd do with any surface temps of sub-instantaneous scalding temp.