Hi,
On the jeemains.nic.in there is a mock test available for the students take that quiz so that you can get an idea of the layout and all the rules of online examination.
Rest there is no change in the exam pattern and also the time period is also the same.
BUT DO REMEMBER TO GET TO KNOW WITH ALL THE RULES AND REGULATIONS OF ONLINE EXAMINATION.
Thanks

Refer the link below for the mock online tests 

good luck and best of luck.

https://jeemain.nic.in/

Online examination is no different from pen paper exam. it's just that here you need to answer the question on your computer screen rather than on pen and paper but there are some different rules for online papers which you need to make sure like in many exams you can't re-attempt or change the answer of your previous questions, you're not allowed to login before the permission of your invigilator. But to prepare for online exam you need to practice online too no? So I am providing you a link, you can go through this link and can practice for online exam it contains paper for most famous ones else you can find another website like this. 

https://testbook.com/courses-practice

hope it helps!!