How long is too long to cook chicken?
Cooking chicken, like anything else, is not about time, but about outcome.

In the case of meat, the most important factor is the internal temperature to which it is cooked.   This should be measured with an instant read thermometer.

White meat should get to about 155-160 F depending on your tastes; dark meat to about 165 - 180 F depending on your tastes.  

Note that dark meat, depending on the cooking method, can tolerate a fairly long cooking time, even when the temperature does not rise:  during that time, the connective tissues (collagen) will convert to gelatin, and it will become more unctuous and almost silky in texture. 

How long this will take depends on the size of the pieces (or whole chicken if cooking whole), the temperature, and the cooking method.  Thighs in a 375 F oven might take an hour to an hour and a quarter; being braised, maybe 30 minutes, for example.

See also:


What makes a moist steak (or roast)?
Temp and times for roast beef 2.3lbs
Is there a general algorithm for calculating the amount of time to roast meat for?