I yet have to do a chucky but did a bottom round roast that was recommended for 2-3 days and the top round at 1-2 days. I did the bottom round at 135*F for 56 hours that was great but we're not comparing the same roasts. I vaguely recall that I read after 50 hours the texture of meat is softer going towards mushy but didn't experience that. I may stick to two days but something came up and being pasturized I got to it when I could.
This is from A Practical Guide to Sou Vide Cooking under tough meat. The last sentence caught my eye.
At lower temperatures (120°F/50°C to 150°F/ 65°C), Bouton and Harris (1981) found that tough cuts of beef (from animals 0–4 years old) were the most tender when cooked to between 131°F and 140°F (55°C and 60°C). Cooking the beef for 24 hours at these temperatures significantly increased its tenderness (with shear forces decreasing 26%–72% compared to 1 hour of cooking). This tenderizing is caused by weakening of connective tissue and proteolytic enzymes decreasing myofibrillar tensile strength. Indeed, collagen begins to dissolve into gelatin above 122°F to 131°F (50°C to 55°C) (Neklyudov, 2003; This, 2006). Moreover, the sarcoplasmic protein enzyme collagenase remains active below 140°F (60°C) and can significantly tenderize the meat if held for more than 6 hours (Tornberg, 2005). This is why beef chuck roast cooked in a 131°F–140°F (55°C–60°C) water bath for 24–48 hours has the texture of filet mignon.
