I took about 6 months to go through all the exams because I felt I needed to understand the concepts for each exam and Oracle itself. I didn't want to have to repeat an exam. There are people at various levels in these forums. Some are new to IT and some are experienced DBA's. Some have taken Oracle classes which prepared them specifically for the tests while others like me have read on their own. Don't take an exam unless you feel you are prepared. All the extra time you spend will help you be a better DBA. Some exams are more difficult - PL/SQL and Performance Tuning - for example. Newcomers to IT who are completing these exams in 2 months may have a great deal of difficulty in the real world of DBA's. Experienced professionals may not need much time because they have worked with these concepts for years. You have to determine your own level. I've read that Oracle assumes an average of one year to complete the tests.
Good luck on the exams.