Hi,

JEE Mains from this year onwards will be held in four sessions. This year they were held in February and March. while April and May session have been postponed. Now, it is up to you, you can either appear for all the sessions or any number of sessions according to your wish. One session usually spans to 3 to 4 days and you will have to appear for any 1 shift in 1 session.

Next in case of JEE Advanced, there are two compulsory papers, that is paper 1 and 2 which are held in one day with a break in between.

Here are the link that you can refer for more details regarding both JEE Mains and JEE Advanced: https://engineering.careers360.com/articles/jee-main-exam-pattern

https://engineering.careers360.com/articles/jee-advanced-exam-pattern

I hope this helped you.