Variable of at least 12 months to 3 years; 2 years is often mentioned by others; The time depends on your intellect, study behavior, free time, family/work or/and other commitments, etc Most people will fail on 1st take, so plan for re-take.
I think all agree that you should start prepping for both Written and Lab together instead of separately. The reason is that there is very little differences in written topics versus lab topics. Of course, you must pass Written first; then lab.
remember Cisco rule: once u pass written, you have 18 months to take a shot at the lab. then 12 months for re-take of lab. if u do keep the rule time frame, then Written exam expires after 3 years. if u do not keep the rule, Written exam is gone and u must re-take it.
Recommended it to pass written when you feel you are ready for the lab then book for the Lab when you feel you are going to pass
of course each person ability to commit/study/learn is different so it might take some people 1 year some would need 1 year and a half, som would need 2 years....
as they say it's a marathon not a sprint....
Easy question, not so easy answer...
Certifications evaluate the candidate based on skills, such tests have tight passing score, which force the candidate to truly know the subject in a theoretical and practical way. This is different than doing a standard degree where students have study time predefined and the passing score is much more flexible allowing the student to commit with a strict time.
As such, I would say, it depends on how much free time you have per day and your ability to study interrupted for a certain amount of time.
But how much time? The time enough for you to feel confident to visit study groups and easily answer any question posted related to R&S, the time enough to open a workbook and be able to do the labs with confidence and be able to explain someone else the content, the time enough to watch videos classes and be able to verify that information is missing in there and be able to write down on paper a small critical analysis of the subject.
It is necessary to refer that work experience on the field and any other certification (ccna, ccnp) would also influence on your study time. Someone with fresh CCNP R&S and few years of experience will succeed faster than someone who is just starting and dare to dive into CCIE.
All said, I will just finish my 2 cents mentioning that the only way you will get this answer accurately is to start your journey and see how it goes... The amount of time people will post here may not be suitable for you for many reasons.
Answer to this question IMO noone can tell you - it is matter of many many personal factors. I am currently preparing my CCIE RS Lab exam so I can tell you my personal aproach to this (if it will help you - super). So, first of all, you prepare your CCIE during your everyday job without even knowing you are doing that Every issue you solve, project you implement, device /feature you configure adds to your knowledge. That you realize when you start doing preparation labs - bunch of known things, already seen tshoot tickets etc. When it comes to real preparation, I believe you should practice as muc has you can - ofc without overwhelming yourself. The pace does matter because you dont want to constantly revert back because you forgot something - and you forgot because last time you did practice was for example a week ago. I personally do something related to CCIE at least couple of hours a day - it is not always configuring, I watch videos, read books or some other materials ... But I am constantly in that "I will become CCIE" mood. Yes, I now have a time limit (due to exam change in february next year) but I simply cannot tell how much time this preparation will take... I simply do it..
So, the thing is that you should not even think about time needed - think about actual preparing plan and daily tasks in order to prepare yourself well for the LAB. You will know when you are ready.