I disagree. As a one off if that 15mg was your first time you would probably feel the same effect within a few days, however doing it like this will build up a tolerance. If you keep it to every 2 or three weeks then the tolerence won't build up.
Please be careful with opiates. I hate to sound like your mother but they really do creep up on you. They did to me and before I knew it I was smoking heroin all day every day, as much as I could get. I'm clean now but it's hard and I would hate to se it happen to you, it's too easy. For your own sake mate, leave them for when a doctor given them to you for genuine pain.