How much hours should one study to crack IIT in class 9?

Class 9th and 10th are important to build your basics strong, Just keep focus on your class syllabus. Don't take to much stress you don't have to study extra for it.

Physics

The topics you study in these classes will help in strengthening your foundation for more difficult and complete topics. Some of the Class 9th and 10th topics in common with JEE Main Physics syllabus are as follows:

Motion
Force and Newton's Laws
Work, Energy and Power
Gravitation
Properties of Solids and Liquids (Matter "? Its Nature and Behavior)
Sound
Electric Current, and Magnetic Effects of Current (extension from Effects of Current)
Optics (extension from Natural Phenomena)
Atoms and Nuclei (extension of Structure of an Atom)

Chemistry

A few of the Class 9th and 10th topics in common with JEE Main Physics syllabus are as follows:

Matter and its nature, and States of Matter
Atomic Structure (extension of Structure of an Atom)
Chemical Bonding and Molecular Structure (extension of Chemical Substances "? Nature and Behavior)
Environmental Chemistry (extension of Our Environment)

Mathematics

Some of the Class 9th and 10th topics in common with JEE Main Physics syllabus are as follows:

Quadratic equations,
Co-ordinate Geometry,
Probability, and
Trigonometry (especially Trigonometrical identities and Heights and Distances).