How long to sous vide a tough cut of steak? With most steaks the recommended cooking time in sous vide is about an hour (of course depending on the thickness of the cut).

I have done some sous vide cooks recently, and they always ended up perfect until last time, where I had a ribeye, which turned out tough and chewy.

I have since learned, that ribeye in particular has some connective tissue, which can make it tough. I have also learned, that most tenderizing methods, like vinegar or baking soda don't actually really work, because they are only in contact with the meat on the outside.

So I was wondering if I could improve the result, if I simply increase the cooking time to say 2h? Or should it be way longer than that in order to tenderize the connective tissue?

What would be the downside of increasing the cooking time?
Collagen (type-1, for the scientific folks), one of the major connective tissues in beef, begins to dissolve into gelatin starting at 55°C (131°F) but very slowly, long enough for the meat itself to become mush.  The pace increases with rising temperature to about 71°C (160°F), at which point further rises in temperature don't accelerate the process much -- but that would be medium-well, a crime for ribeye.  Unfortunately, collagen begins to denature at 68°C (154°F), which is also when the meat begins to constrict/toughen substantially and release lots of its juices.  Cooks Illustrated suggests 54°C/130°F for 2-3 hours, but in my steak that didn't melt the collagen at all.  I find the best trade-off is medium, 63-64°C (145-147°F) for 3 hours. Yes, it isn't the deep rosy and extra-tender medium rare, but most of the connective tissue vanishes and is replaced with luscious gelatin.