Stewing beef - how long is too long?

I'm going to assume that you are covering your meat with liquid, because if you aren't that's the problem. Collagen needs heat plus water to break down, if you don't have liquid it will turn tough as old leather.

There is no point where meat gets tougher when being slow cooked, what happens first is that the collagen breaks down, then the proteins start to denature. Generally once the collagen is all broken down is the ideal time to take it out, any longer and the meat itself starts to break down.

Things to remember:

• How long it takes for meat to get tender in a braise or slow cook method depends on the size of the cut, thickness of the cut, and the amount of collagen in it. Working cuts like chuck, rump, neck, shin, etc have lots of collagen and need a longer slow cook to break it all down. They also have more flavor. Non-working cuts like filet, sirloin, etc would need much less time, but to be honest you're wasting good money slow cooking cuts like that, roast them instead of grill them as steak! Bigger cuts and thicker cuts will need a longer slow cook as well as it takes longer for the heat and the moisture to get to the center
• Some meat will never get tender no matter how you cook them. Low-quality, poorly processed meat that has been frozen and then quick-thawed is likely to be tough as old leather no matter how lovingly you treat it in the pan. It's the GIGO principle, Garbage in-Garbage out, you can't make bad meat good with technique. So it may not be that your meat toughened up after slow cooking, it may be that it never had the potential to be tender in the first place!
• Moisture is critical with working cuts as it breaks down collagen, in a slow cooker the meat must be completely submerged in liquid. In the oven it's important to have a lid on your braising dish, or at least tin foil.

You have some good answers here. I would suggest the following.

Always have plenty of liquid for slow cooking any meat. You can ensure this by cooking in a covered pot/wrapping in foil if in the oven or keeping the lid on for a slow cooker. Placing you meat on vegetables such as carrot or onion will add flavour and moisture. Look at it after an hour or so to make sure its not drying out and top up if required.

Secondly choose your meat - hard working muscles will develop more fibre so need slow cooking to break then down but that will also release the flavours. Cook slowly at lower temperatures 120 c for 4 hours or so for a regular family sized joint (longer for a larger item). Then for a further hour finish uncovered in a hot oven which will brown/crisp the outside. Test meat with a fork for tenderness so you don't over cook/dry it out. Let your meat stand/rest for at least 30 minutes before carving - it will absorb moisture back in making for better eating.

Hope this helps

There is a good guide to the different joints of beef and cooking methods here

Cooking meat does not make it tough, it gets more tender.

Meat does get dry at higher temperatures, time does not matter that much.

The higher the temperature the more moisture is squeezed out of the meat making it more dry, I guess that it what you describe as tough.

See Baldwins write up at http://www.douglasbaldwin.com/sous-vide.html#Effects_of_Heat_on_Meat it looks like the moisture loss increases most at 60-65C/140-150F. A slow cooker have much higher temperature than this.

This is also a good answer http://www.douglasbaldwin.com/sous-vide.html#Tough_Meat, i.e. 100 hours is not to long!

I use a slow cooker to cook beef all the time. The general rule of thumb is that a high temp setting usually finishes cooking the contents in 4 about hours, a low temp setting is usually finished in about 8 hours. I've never had an issue with beef becoming tough in a slow cooker. Usually quite the opposite, it's so tender it falls apart. In the oven, you can also slow cook beef, or any other meat for that matter, by adding a liquid (water, broth, beer, or wine), covering the pan with a lid or foil, and setting the temp to 225 fahrenheit (107 celsius). Which ever method you choose, turn the meat every half hour to keep the top from getting too dry. I've also found that if you pan sear the meat before cooking it locks in the moisture for a more flavorful meat. Hope this helps :)

I would like to politely disagree with the "cover w/ liquids" camp. No liquid is required to slow cook a beef chuck roast. Even if you are using it to make stew, as it will make it's own liquid. You can even short-cut and skip the searing (gasp) if you are so inclined. I grease crock lightly and slow cook two small grass-fed chuck roasts (2.5 lbs each) side by side in a Hamilton oval "travel" slow cooker. The lid has a thermometer hole and a plastic seal that I dislike, so I cover w a silpat silicone baking mat, lock the lid down & set on low for 7.5 hrs. Season, but do not salt. Disregard advice to turn meat periodically unless you are prepared to add additional hours of cook time. Taking the lid off defeats the purpose of using a slow cooker in my experience. Look for finely marbled cuts for best texture and flavor-you can always discard the fat. If you are using grass-fed, hormone-free, steroid-free beef ingesting some of the fat is good for you. Sorry, but there is no one size fits all answer about a specific time before which or after which any cut of roast is tender. Slow cookers are variable. Your cut, the cows's diet-all variable.

I am cooking a roast right now on the stove. I seared it with onions, garlic, oregano and olive oil however I made the mistake of not completely submerging it in liquid, I just added a bit of stock at the bottem. I figured the steam will be enough liquid to cook and still be soft, I was afraid more water would take away flavor but it doesnt if spiced correctly. After I sampled the roast it was tough so I decided to add water to the beef stock so now it is completely submerged on a low temp and the good news is, it is softening up. It needs to be surrounded by water to break down the fibers of the meat, steam is not enough on its own. I never had a problem over cooking the meat, its only tough if its not surrounded by mosture, thats the difference between pot roast and roast beef. Beer and wine are amazing at tenderizing meat and cook slow if not in a rush. I would have used a crock pot which comes out perfect every time but my husband was hungry.

I disagree with the comment that the slow cooker does not dry out meats. I have to slow cook meat to breakdown the fibre due to digestive issues and both the sirloin and chuck cooked in a fair amount of water w salt after 7-8 hours on low have dried out the meat completely. This week I decided to slow cook each steak for 6 hours and the meat was completely tender. Big difference. And there is w a longer cooking time.