Hello

There is weihtage given to work experience in all IIMs. So having a work experience gives those extra marks that help to get the final conversion in the IIMs. 

Genrally the work experince follows a bell curve. For example it will peak at around 24 months for most colleges and for some like IIM C it peaks at 36 months. So ideally 12-24 months of work experience is the best slot to get admission.

The statistics for top 3 IIMs to show that selection process is not at all about work experience, but your overall performance in academics and CAT entrance exam. It's not required to have work experience to get in