How long to sous vide a tough cut of steak?
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.