Hello Aspirant, 

You should attempt mock test after you complete each unit, and only for that perticular unit, once you complete half your course start attempting mock test twice a week, with complete Time management. Due to low sperm students are not able to attempt whole paper of JEE, missing some of the easy questions. Also register yourself in some test series, because they provide you fresh questions and fresh material. Do every possible mistake in the test, they will improve you to your best. For the final paper, don't waste your time on the question you think might take time or for the question you aren't sure about, that's also a common mistake people do.

All the best, if you have any further queries then do ask.

Yes, it happens with many aspirants. Mock tests are generally a bit higher level than the exam, to prepare you well for tough problems.

Sharing some tips on how to attempt questions. (HT - Anmol Thakkar, IIT Kharagpur)

Answering JEE Main Paper in 3 Phases

• In the First Phase, do the easy questions. Those questions which you are confident of and sure that it will not take more than 1.5 min strictly. Skip the Hard & time consuming questions.At the end of first phase, If you had a decent preparation then the number of questions you have answered gives you the average score. Remember those were the easy ones and most of the candidates can answer them.
• In the Second Phase, take up those questions which you could not answer in the first phase. The questions might be time consuming (3 min). Might be a tricky word problem which you could not crack in the first phase. For all such questions you have the second phase. (Intermediate Questions)Your performance in this phase will help improve your rank. So focus and Concentrate. Try to resort to methods like Elimination of Options by methods like substitution. Do not always go first for conventional solving method there is always an easy way.
• In the Third Phase, take up very difficult and time consuming questions. Answering them correctly will improve your rank drastically. Questions for which you have no clue since you have either forgotten the formulas or do not know the concept should be dealt in this phase. Risk Factor comes into picture here. Make a smart guess of the option if you feel it is necessary otherwise there is no harm leaving them and start checking the previously answered questions.

Hey there

For JEE main for mock test of full length you should start from December last if you have completed. You should give as much test as possible and find your weak portions and do strong that portion. As mock test is the biggest tutor. 

Check about JEE mains

https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/exams/jee-main&ved=2ahUKEwjP67XE_4vjAhWWV30KHQ_5DdYQFjAAegQICRAB&usg=AOvVaw1k3rMyr-Osb0qy4gsPfOaR

Good luck