time of cooking

That was a pretty vague question, because the type of meat wasn't even identified. But for meats, a given temperature defines whether the meat will be rare, medium rare, medium, etc. once that temperature is achieved, dwell time determines the level of pasteurization and the degree of tenderizing performed. Different meats require different temperatures. Fish is cooked to low temperatures at which no pasteurization occurs and the longer the exposure to heat the more bacteria is grown. This means all fish gets very minimal time in the water bath. Cooking fish by more traditional means has all the same problems, except that searing reduces the bacterial count on the surfaces being seared. You can sear fish after cooking sous vide, too. Let it cool a bit before searing. <br /> <br />Most foul (chicken, turkey, etc.) should be cooked to medium done for white meat or medium well for the dark meat. This means 140 degrees for white meat to 148-156 degrees for dark meat. Foul is naturally tender, so cooking times are kept short to prevent turning the meat into mush. Some foul is all dark meat no white meat, such as duck, and some foul recipes such as dick confit require special temperatures and cooking times to achieve the desired texture. <br /> <br />People are used to getting pork loin cooked until all pinkness is gone--this is a medium 141-142 degrees. The naturally tender parts of pork require less cooking time than the more exercised cuts such as the ham. <br /> <br />Which brings us to beef. There's a lot of variety in beef cuts, some are naturally tender, some naturally tough. I recommend careful temperature control and 18-24 hours for the lean and tough cuts at precisely 131 degrees F. This includes round steak and London Broil, and almost every cut of beef down to just short of ribeye and New York steak. The ribeye and New York are, in my opinion, best cooked at 131 degrees for six hours. Extra tender cuts like tenderloin should be cooked at 131F for about the minimum time that the thickness dictates. Otherwise, if you don't want to go to the trouble of figuring this out, go with 2.5 hours for all but the thickest cuts. You may get mushy results, but more than likely you won't have that experience -- that usually happens only when the meat is aged for great tenderness already.

Mostly by a) science and b) experience <br />For a given time for a given food (let's say meat). If you cook meat to less than the lower range it will be tough, or maybe (for short times) not even cooked to the desired done-ness. If you cook meat to more than the upper range the meat will still be cooked, but it will begin to turn mushy after that. At the beginning, target the middle of the range, then adjust appropriately... the beauty of the water immersion method is that an hour more or less will not make a lot of a difference!

You must understand what sous-vide is about, the differences between tender and tough cuts, that times to reach temperature depend on shape and width (and that there are tables for these), and that times and temperatures also depend on the security you want (how much patogens are killled). <br /> <br />Then go search for decent recipes, one that gives a broad range without explaining why is not a good one.