I have stage iv lung cancer is it common to take chemo for 2 years?

It is possible: For a significant period of time the standard approach was to treat for 4 months and stop, because studies showed that ongoing treatment was no better than restarting when things progressed. Recent data show a survival advantage with Alimta or Avastin (bevacizumab) after 4 months of chemo. It makes sense as long as you are tolerating it. I would discuss your particulars with your oncologist.

Yes: Most patients with stage IV ling cancer, both small cell and non small cell, will be on chemotherapy for a long period of time. The regimens may change and be less intense with maintenance therapy.