Generally speaking, large sturgeon can withstand an extended fight and suffer no detrimental effects from the battle. Even after a prolonged fight, like you, I've seen many large fish swim away with an excess of energy and appear in very good health. As far as i know, there has been next to no mortality documented from sport anglers catch and release of these excessively sturgeon. With saying this, we still have to take care of how we handle these fish and ensure we give our best effort to reduce the amount of time the fight lasts and handle them accordingly. As a precaution, efforts are now being taken to reduce fishing pressure in pre spawn locations. This is being done to ensure that if there is any negative stress on these large pre spawning females, they do not abort their eggs.
5-7 hours seems extreme in the length of time needed to land even a 10-12 foot fish. In my opinion, if the fight is going beyond a few hours (3-5), there is likely other factors leading to the excess time needed. The fish is likely fowl hooked, angler fatigue, or the tackle is to light for the size of the fish. Yes, the fish could be larger than 12.5feet, however there has yet to be any documented bigger in the tagging study.