While I've seen only a few bars (both drops and flat) that have had cracks near the stem (including working through the 1990s Specialized recall) these were all still in one piece. It was during the service of the bikes that these cracks were found (and there's something to take away).

The few I have watch break during riding came quite quickly. But all of these were drops that had cable routing holes drilled in them and rather crudely at that. Two with the same rider who, while "sprinting' away from a stop, simple found the rt side of the bar in his still rising hand. He basically did a dive down to the drive on the left.

So to answer the question as to the rapidness of a stress riser giving out completely- I think that the answer is an opinion. In that if the rider had no knowledge of a started crack, the failure will be called "instantaneous". And if the rider regularly looks for the signs of failure he will say there's almost always an indication of the pending failure. And that's the other take away, check your equipment periodically! Andy.