Progression free survival 2.2 months
Since you are having stage 4 lung cancer
and if your EGFR mutation
is positive in biopsy sample then you will respond well to erlotinib
The drug will keep your tumour under control, palliate your symptoms and will improve your quality of life.
Since you are already in stage 4 of the disease progression free survival with tarceva will be around 2.2 months and overall survival will be 6.7 months on an average.
Hope I have answered your query.