Im myself preparing forR/S but i can cite an example of my friend who always used to carry his resignation letter in his bag 
. he told me that one must devote maximum time to reading the material before thinking about quitting the job for the lab preparation since according to him the last thing one wantes to do is to spend time in reading specially when he/she is jobless.
He did 3 CCIEs in 2 yrs and i think he was kinda right in his estimation.
Coming back to your query. as Paul/Scott rightly pointed out that it depends on individual to individual and that you must devote time to prepare for the exam in order to be able to become a CCIE and not someone who is preparing to pass the exam. (JD) since in that case you;ll have a much better chance of clearing the exam in 1st go. and i also believe that CCIE Service Provider is a close ally to R/S so if you can study the MPLS portion (not entirely) from the SP books then you can gain an upper hand and clear that (SP) exam in a jiffy.
So devote max time to reading the material and gaining knowledge in all the core topics and subtopics of CCIE R/S
Finally, Follow the simple rule
Read->Configure->Break->Troubleshoot
Resignation letter from Microsoft Employee http://tinyurl.com/2439jkx
Writing my resignation letter. Can’t wait to get the HELL out of this place.
wow! done it! just sent my resignation letter!
Agree! Cc: Dear Roy, we hereby request you to submit your resignation letter in the (cont) http://tl.gd/6j1toq
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Typing up a resignation letter, chunkin the dueces at the Deaux!
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