My sous vide chicken is tough and stringy - did I cook it too long, or too little?
You cooked it at too low a temperature. 

Sous vide is intended for meat where you want the protein to remain tender. It shouldn't have any sinews. Think chicken breasts, or the long filet along the spine of a pig. This meat gets nicely cooked at 60-65°C (depends on the animal), and tough and dry above that. 

Meat marbled with sinews has to be cooked at a temperature where the sinews (collagen) melt into gelatin. This happens at about 70°C at least, and takes hours. Since the muscle fibres are already toughened at that temperature, there is no reason to hold it low; you can put it at full boil in a normal pot and cook it there, you just have to wait long enough. In theory, you could do it in a sous vide bath too, but you won't get any of the benefits sous vide gives to tender meat.