While I agree with your title, I feel compelled to answer.
First of all, is this something you are considering? I don't know you or your story, but this is a drastic step.
As for how long, it is impossible to know, everyone is starting from a different point; however if your FEV is 40% then you may be well down that road.
Ultimately, what will get you depends on your CF issues:
If you are signficantly PI, then you will drop weight rapidly, your vitamin levels will drop, you will become weak, and susceptable to infections. In addition, you could develop a bowel obstruction. Both the vitamin issues and the bowel obstruction could, if not treated, become fatal.
Assuming you follow a more 'normal' course, the lack of lung treatments will allow any bugs you currently culture to grow ramptant. Plus, you will become an open door to new infections. You will ultimately develop pneumonia and your FEV's will drop, likely quickly w/o treatment. As your lungs fail you will ultimately lose your fight with CF.
As I said, how long is up to God. You could live quite some time or you could last weeks/months. A lot depends on your current overall health and what you are culturing currently.