From the recent Career Certifications: ASK THE EXPERT – PREPARING FOR CCIE IN ROUTING & SWITCHING with Maurilo Gorito (Maurilio is the content manager for the CCIE R&S exams):
Just a basic question and i know that it differs from person to person, but based upon your experiences how much time (in hours) should it take for preparation of the CCIE Lab. I heard one CCIE say it took him over 2000 hours… This is presuming the person was already a CCNP.
Yes, it will vary from person to person, the time you have until your lab exam, how many hours you can allocate daily just to study and your level of hands-on experience.
Considering the above factors I would say after you have passed the CCIE R&S written exam and dedicating 20 to 24 hours per week having hands-on practice applying what you have learned and your experience, I believe you will be well prepared to take the lab exam in 6 months.