Now I am in my 5 th semester .7 semester I am having my placements.when shall I start my aptitude preparation ?
You are in your final year of engineering. So you can start with the placement interview preparation. You can start reviewing your things leant, enhance your skills, gain some experience (if possible through internships). You can start your aptitude preparation as this is a decent time.
Also be prepared with the questions related to your specialization of your interest as they may ask you the reason to choose it.