Hi,
Most of the IITians start preparing for the placements just six before these. Some of them start one year before of these. Few start preparing as soon as possible after their admission. IITs have course credit system in which you have to complete the credits within 4 years. But you are free to complete in 3 year too. Many students do it this way. They complete their course credits by the end of 3rd year and keep their 4th year almost free to utilize it for the placements or their endevours.
Most of the time IITs courses are tough to understand as well as score. This motivates the students to form the various groups for various subjects and activities. This pattern remain unchanged till their placements even after IIT days. This is one of the biggest reason of their success. One more important thing to make this happen is the challenging environment of IITs which pushes their egos, attitudes etc towards each others, in unknown dark corners.