Hi,
I assume that you are going to prepare for JEE 2020. On a avergae if you spend 4 to 5 hours a day. You can easily complete the syllabus of both 11 th and 12 th. Start with 11 th syllabus  simultaneously refer  to similar or higher concepts in 12 th syllabus which have reference to what you studied before. Side by side you can cover some of concepts.  Then complete the remaing.
Your preparation doesn't depend on how much time you spend. 5 hrs is the rough time i have suggested.  If you are already clear with your concepts or basics . Then it won't take much time to understand 11 or 12 syllabus. For average student , if he studied woth thorough understanding and practice,  one can a answer almost 70 % of JEE exam questions and rest 30 %require extreme practice and serious preparation accompanied by repeated small small practice tests and revisions.
If you spend average of 5 hours per day and if you are already cleared with basics till 10 , then you can complete the whole syllabus in 5 or 6 months , spend the remaing time for practicing and reviewing your basics.

It depends..... If you are aiming at top IITs you need to give a lot of time and you have to study constantly but if you are aiming at some lower IITs and NITs then you just have to study your 11th ajd 12th course and solve some books like HCV, RD sharma and NCERT is enough for Chemistry but you will have to be very precise and solve last year papers.No one can tell you how much time it will take but if you study honestly than you can easily finish 11th and 12th course for IIT in 1 year time period.