It all depends how focused you are. I took 4 weeks off work, put in 12 hours per day completing a DL syllabus and smashing every question bank I could get my hands on (but mainly BGS) and took my first 4 ATPL exams in the 4th week. Then I went back to work for a month (to relax!) and did the same again the following month and repeat... I wouldn't recommend it but it helps being self employed. I basically studied 6 days per subject.