I (Dr. John Townsend) have talked to many young adults who have told me, "When I was a teenager, I acted like my parents had nothing to say to me. I couldn't afford to act differently. But inside, it mattered a lot what they said."
You can't predict how telling the truth and establishing healthy boundaries will affect a teen, nor can you predict when the change will occur. I have seen parents with a seventeen-year-old who would be moving out in a few weeks still make significant inroads with a rebellious and destructive attitude. Don't let your fears and discouragement limit a process of growth that God designed for your child. Sometimes the right intervention, given at the right time, with the right people, can make all the difference in the world.
Even so, Let's suppose you do have a teen who is not doing well and is almost out of the house. Consider the alternative. If you give up and go into survival mode, your teen has not experienced the benefit of being around loving, truthful, and strict parents and will be that much less ready for successful adulthood. Even if your teen resisted every effort you attempted and you saw no change at all, something good has still happened. In those last months and weeks, she has experienced and internalized some events that cannot be easily shaken loose. For that brief time, love, responsibility, freedom, and consequences were applied to your teen's life in a way that was healthy and good.
As a psychologist, I have met many adults who blew off their parents' help when they were teenagers, only to remember years later what had been done. And they know at some level that that was a good way for them to live. So even if you don't see the fruit today or tomorrow, your teen will still have some memories of the way life should be lived. Take encouragement from the words of the prodigal son in the Bible who finally "got it":
"When he came to his senses, he said, 'How many of my father's hired men have food to spare, and here I am starving to death! I will set out and go back to my father and say to him: Father, I have sinned against heaven and against you. I am no longer worthy to be called your son; make me like one of your hired men.'" (Luke 15:17-19)
Don't count on getting an apology like that. Instead, fight the good fight of setting boundaries – all the way to the last minute that your teen is in your charge. Your investment of time and energy will not be in vain.