The dude who sells me dope shoots H into his ankles and feet just so there won't be any marks on his arms. He says it's more difficult to hit but he's gotten good at it.
As far as how long it takes before someone takes up the needle, that depends on the individual. I know one lady who had a decades long addiction to H and still smoked it. I also know one guy who started H with the IV ROA, so it all depends...
Its possible to shoot in your feet and ankles but I dont believe any serious addict is going to be able to maintain that for very long. My friend got to where the only place he could shoot was the vein next to his penis, so he would have to drop his pants every time we used, which I always found hilarious but no one else seemed to see the humor in.