You need to give equal time to all the three subjects they are equally important in qualifying the exam.
All i would suggest is analyse the important chapters and topics of all the three subjects. see which topics are given more weightage to, and then focus on those topics more and practice questions related to those topics more.
Keep on taking sample papers and previous year papers for practice and also analyse your result to improve your performance.
Every subject must be given equal importance to score good marks in entrance exam.
As the JEE Mains paper consists of Physics, Mathematics, Chemistry with equal number of questions that is 30 questions for each subject you should give equal time to each subject.
If you still have any queries feel free to ask in the comment section down below.
JEE Main Sample Papers
JEE Main Previous Year Question Papers with Answer Key