Welcome to the forum.
There's great information on ribs and other meat to be found in Smolinokie's 101 series, found
HERE thread.
"Falling off the bone" is a function of heat and lenght of cooking time. Many of the forum members rely on the toothpick test to determine doneness. Placed through the meat, between the bones, when a toothpick slides through with ease, the ribs are considered to be done. This is usually indicative of a "KCBS" bite thru, which competition judges look for.
Prolonging the smoke process once the toothpick/tender test is achieved will further soften the texture of the meat and in doing so, the meat will release from the bone easier.
Simply stated, smoke the ribs until one of the end bones tugs away cleanly. The time will depend on the weight of the ribs and temp of your smoker. A word of caution, if the ribs are lean to begin with, you may wind up with dried out falling off the bone doneness. One workaround is to foil the ribs (with or without sauce) one they hit toothpick tender. This will not only quicken the remaining time need to further tenderize, it will help prevent moisture loss.
There is a LOT of information regarding pouched ribs in
THIS thread. Hope this helps.