hot load makes the bolt sticky

really hot load makes bolt very difficult to beat open

Error, equipment failure, or major mistake by operator makes bolt next impossible to open and splits stock.

ow close to failure is the metal now

How much damage to the metal that can't be seen

When will it fail

Will it fail

Fortunately, with modern steels, it is very hard to place enough stress on the receiver or barrel to create a weakened situation that will fail with standard loads. It may stretch but tends to stretch but retains tensile strength. It it sort of a self regulating system too, the primer will fail and gas pressure will be released long before the steel is subjected to very high pressure. (There are exceptions of course, especially if the steel has been improperly heat treated (too hard is much worse)). If we had steel primers, screwed into a steel case, we could be shooting 100 thousand PSI all day. Tank cannons to just that.
So the answer to your question is "never" at least not after a blown primer.

The first part of the action that will show damage is usually the bolt. Often the damage is caused by "hammering" the bolt open instead of removing the barrel so the swelled case can be removed from the bolt. Sometimes there is measurable "set back" in the lug area of the action. I have had a Sako action where the cocking piece was damaged from a blown primer, the little lug that holds it into the action sheared off and was about a sixteenth of an inch from coming completely out of the bolt and sticking into the shooters face.
Usually the actions I see are the ones that the customer is either unable to remove the case or has already bent or broken off the bolt handle.