Electrostatics is one of the most important topics in Physics for the JEE MAINS & ADVANCED examination. Now the problems, from this topics are usually mixed with Mechanics. So you should get a good knowledge of mechanics mainly Rotational Motion, SHM etc before you start doing the Electrostatics. Before problem solving you should read up the theory well and do some broad answer type question so that you can really analyze the skill required for the subject. So if you are up with a good theoretical background in Mechanics as well as Electrostatics itself then you can start solving problems. In solving problems of Electrostatics, the Vector analysis and the use of Integral & Differentials will be of great use. So before starting also try to revise the mathematical concepts as well.

The ideal time to do problem solving is dependent upon you. You can do it whenever you want. I would advice you to always do practice problems during the examination slot timings, so that in those hours you brain should get settled to do calculation stuffs so that while you will be up for the actual examination, your efficiency will increase.

For more information regarding JEE MAINS preparation tips consider the link below:

https://engineering.careers360.com/articles/how-prepare-for-jee-main

For more information regarding JEE ADVANCED preparation tips consider the link below:

https://engineering.careers360.com/articles/jee-advanced-preparation-tips

I hope this helps. Wish you a great day!