Place the ribs, uncovered, on a wire rack over a baking sheet and place in a 225 degree F oven for up to 8-9 hours depending on the thickness of the ribs. See Note.
After making these ribs quite a number of times, I would say the 8 or 9 hour cooking time is for only the largest thickest racks of ribs, like side/spare ribs or large St. Louis style ribs.
Back ribs in my experience can also vary in size a lot. Baby back ribs might take 4 to 6 to hours for example, whereas a large rack of back ribs might go 6 to 7 hours.
When you twist one of the bones and it begins to break away, then they are cooked well. I like them at a stage where the meat is very tender, the fat is well rendered but you can still easily cut them in individual ribs and eat them off the bone without them falling completely apart. I have been known to slice a test rib off the end on occasion. :)