spares or baby back? the esoteric answer is the best. they are done when they are done! I have seen people say as much as 7 hours and have baby backs done at 3. depends on the fat,how much meat is on the bone and that days running temps.Did some ribs in a new smoker and not 100 % satisfied. Had them in a GOSM 3045bgw at 225 for 5 hours they were cooked but not fall off the bone done. How long should ribs cook for?