Well, I can't specifically answer your question, but I can surely tell you that time wise a 'Ketonic' diet is all based on the dieter. I personally would not run it for more than two weeks without a carb-up. Now, my usual way of running a ketonic diet is as follows:
12 days in ketosis-> 2 day carb up
10 days in ketosis -> 1 1/2 day carb up
7 days in ketosis -> 1 day carb up
-> 1 week break where I take 1 day to eat everything I want, and the rest (6days) eat high protein - mod. carb - low. fat
-> 14 days in ketosis > 2 day carb up
" ^ " then just as the above schedule
You have to understand that this is not for everyone. I am very "extreme" when it comes to dieting. I do things to the dot, no more/no less. So, if I start a keto diet I plan it out all ahead.