Howdy, Gina! Vision, the rule of thumb I use is (and this is on a sliding scale, depending on if you steam them with beer or other marinade): when the meat starts pulling back on the bone 1/2"-1" and the rib tips are exposed, it's time to start with the BBQ sauce, more dry rub, or whatever you prefer. Once the rack gets that crispy bark along the edges, it's dinner time. Some people like a little more bark on their ribs than others; I prefer the former as long as it's not turned to charcoal. Again, your mileage may vary - different regions of the country have their own unique way of smoking, the ingredients vary, the thickness of the rack, and so on, and it's a matter of regional pride and bragging rights because
everyone thinks 'their way' is 'the way'. LOL Over the years, I guess I've appropriated a little of the best of everyone's methods, used trial and error, and hit on my own specialty. But, back to your question, when the tips start to poke out and the bone is browning, you'll know your rack (or spare ribs, if individually cut) is about halfway done.
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