The best time to start preparation is at the beginning of 11th class. Students are fresh after the break after boards and a lot more can be learnt. Students who start late often end up leaving some key topics of 11th class JEE syllabus. Also, if you start your preparation in 12th class then the amount of hard work is increased manifold due to additional pressure of 12th boards, so chances of getting a high rank in JEE are decreased. There is a big transition from the mugging up study of 10th class to more understanding based study of 11th class. So you shouldn't get bogged down by seeing the level of complexity in JEE exam.
Course planning is essential in the sense ,that it can get you at the top in rank list.This planning should be done by the student in 11th class itself . The summer vacations in 12th class are the best time to finish a huge lot of syllabus.I can suggest the following course plan for students in 11th class.
JUNE TO OCTOBER : Mathematical concepts used in Physics, Basic mechanics, stoichiometry, general chemistry----atomic structure, chemical bonding, periodic table, algebra---number theory, quadratic equations, progressions.
OCTOBER TO JANUARY :Complete Mechanics, waves, Physical Chemistry--equilibrium, heat, kinetics, thermochemistry, electrolysis, algebra -----induction, complex numbers, trignometry.
JANUARY TO MAY : Heat, fluid mechanics, gravitation, Inorganic chemistry---properties and metallurgy of metals and non metals, gas laws, analytic geometry, permutations and combinations, binomial theorem.
MAY TO AUGUST : Electrostatics and electricity, electrochemistry, solutions, solids, qualitative analysis, transition metals, differentiation and integration.
AUGUST TO NOVEMBER : magnetism, optics, modern physics, organic chemistry, differential equations, probability, vectors.
NOVEMBER TO DECEMBER : FINAL REVISION
By following this plan,course will be finished in time and revision can also done,to ensure maximum success in JEE.To maximize your potential you have to planned .