It took me lot of time to figure out the apt time to start preparing for the placements but I figured out well lately.
The best time to prepare for the placements is the start of 3rd year (never ever wait till final year and obviously not first and second year because it is too early for serious stuffs 😛 ). So in 3rd year there won’t be enough rush for anything and you will have enough time to plan out your preparation. So a head start from 3rd year is always advisable.
Aptitude and Comprehensions takes a lot of time so starting it in the final year is not a good idea as there will be lots to do like final projects, presentations, companies coming up every other week so its will be all rush hour and frustrating if you cannot manage enough time to prepare.