It can take a while. Depends very much on you. It will drop to some extent over a period of weeks, but it won't come back to zero. It's debatable if it ever comes back to base levels before you started using even after years of no use, though it gets damn close, but it might take you some months to get somewhere even approaching base level tolerance. The effects of opiates on the brain are perhaps semi-permanent then IMO. Your brain will always remember what opiates are and what to do with them, evidenced by the fact that tolerance reestablishes itself very rapidly once you've had a habit, as does addiction. As others said be cautious, because you're not gonna know where your tolerance is after a period of abstinence until you hit it with opiates. Assume it will be low and titrate dose accordingly.