Old thread but maybe some new advice. Ribs are catagorized by weight. The smallest I know of are babybacks and they are 2 1/2 & down, meaning their weight is 2 1/2 pounds or less. My experience is they start getting tender at approx. 3 hours at 225. Cooking a little longer will produce a ‘fall of the bone’ product, which you may or may not want. (Don’t think BBQ judges want this, but my customers sure do.) I sell a St. Louis cut that are 3 1/2’s & down and they take 4.25 hours at 225 to reach the ‘fall off the bone’ stage. Just check the weight and you can pretty much predict when you should start checking them for your desired tenderness.