How difficult is it to get a branch change in IIT Mandi to CSE? How many hours of study is sufficient?

The difficulty of getting a branch change in IIT Mandi to CSE depends on various factors like the current demand for the CSE program and the availability of spots in the program. The branch upgradation policy at IIT Mandi is very easy making it highly possible to get a branch change to CSE if you start working from the beginning. All you need to do is maintain a stable CGPA around 9. The 50 % branch change criteria make it easy for everyone to get a branch change. The increasing batch-size changes the cutoff every year but to play safe, maintain a 9+ CGPA. You will be surprised to know that some even get to join CSE with a CGPA of 8.6+. 

The number of study hours required to get a branch change depends on the individual, his intellect, and his learning style. The general rule of thumb is to study at least 3 hours per day for undergraduate courses.

All you need to do is be consistent with your studies and attend classes regularly. So, it totally depends on what works for you.