One of the most common questions I get asked about my sous vide recipes is some variation of "the recipe says to cook it for 3 to 6 hours, but when is it actually done".
As long as the food has been in the waterbath for more than the minimum time and less than the maximum time, then it is done. There isn't a specific magical moment of true doneness that can be generalized.
For those that want more information, here's the explanation why.
To have this conversation we first need to determine what "done" actually means. For sous vide there are two main "doneness" concerns when cooking your food.
The first is to ensure that the food actually comes up to the temperature you are cooking it at. Or for some foods like chicken and poultry, to ensure it stays at that temperature long enough for it to become pasteurized and safe to eat.
The second concern is making sure the food is tender enough to eat without being "over tender", mushy, or dry.
For some already tender cuts of meat like filets, loins, and chicken breasts you don't have to worry about tenderness since they start out that way. That means that these cuts are "done" once they get up to temperature and/or are pasteurized. You can find out this time using my Sous Vide Thickness Ruler or reading my article Sous Vide Cooking Times by Thickness.
However, despite them being "done" at the minimum time shown, they stay "done" for several hours past that time, depending on the starting tenderness of the meat. This is why I give a range. You can eat a 1" cut of filet mignon (25mm) after 50 minutes but you can also eat the filet up to 3 hours after it has gone into the bath without any loss in quality, tenderness, or flavor.
This is how my ranges are determined. They specify that for an average cut of the given meat, they will become "great to eat" tender at the minimum time given. They will continue to get more tender the longer they are in the bath but will remain "great to eat" tender until the final time given, at which point they may begin to get mushy and overcooked. In essence, they will be "done", and very tasty, for that entire span between the minimum and maximum times.
Another way to think about how this works is to use the following analogy. Pretend you were helping a new cook grill a steak. If they told you they wanted to cook it medium rare and asked you to tell them how to tell when it was "done", what would you say?
Most people would reply with "when the temperature is between 131°F to 139°F" (55°C to 59.4°C).
If the friend isn't a cook they would ask "Yeah, but when is it actually done?"
The answer at this point really comes down to personal preference since to some people medium-rare is perfect at 131°F (55°C) and others prefer a little more well-done 135°F, (57°C) but a medium-rare steak is "done" anywhere in that range.
One other complicating factor is that there are many variables that go into determining how fast a piece of meat tenderizes and/or becomes tender.
The most obvious variable is that some cuts of meat are tougher than others. For example, a top round roast needs to be tenderized a lot longer than a ribeye. Most people realize this and that's why almost all sous vide charts break the food down by "cut".
Another less obvious but almost as important factor is where the meat came from. There is a big difference between how fast the meat tenderizes and how the cow was raised. I've found that grass-fed meat from my local farmer needs just 1/2 the time to become tender compared to supermarket meat (this is also true when roasting or braising them). I've also talked to a reader in Mexico who eats local grass-fed beef that needs slightly longer times than normal because the cows work more.
There are then the variables in the actual cow itself. Whether the meat is prime, choice, etc. makes a difference in tenderizing time. As does the marbling, how old the meat is, and several other factors.
So taking all of this together it can be hard to accurately determine a range of "doneness" that will work for all cuts of meat. But I try my best to come up with a nice range of times that the "average" piece of meat will be done in. The only way to really learn is to experiment with the types of meat in your area and see how they react.
So while there might be one magical moment in the cooking process where a certain piece of meat is the most ideal tenderness, in practice there is a wide time range in the cooking process where the meat will be "done". As long as you take it out some time in that range it should turn out great.