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.