How the work experience play a role in selection process and what can be maximum months as work experience?

Work experience is IIM's and other bschools is not a mandatory thing. Even if a student is a fresher the preference can be given to him if he has scored good and has made a mark in interview rounds.

Below 2 years is not given any weight-age generally. So keep the scale high. Also the BPO experience is not being considered so it should be related or relevant.
For work experience some bschools rate out of 8 and others out of 10 that is the total weight age.

Though it is recommended to have a work experience before joining a MBA program so that it not only adds your value in the selection but after you complete your MBA it keeps you on a hike from the start.

Hope this explains you well enough.
Good luck!

Work experience carries a certain weightage in almost every b-school interview and selection process.  When I gave cat I remember that different marks were given based on the number of months of workex. It usually increased as the number of months of workex increased and then after a point started decreasing. For example you were awarded marks which kept increasing till say 48 months(4 years) of experience and then started decreasing. The best workex according to me is between 24-36 months. This way you are shortlisted for all kinds of companies. Sales and marketing and finance roles generally shortlist freshers where as consulting, IT and operations roles consider people with workex. For general management they consider both freshers and workex people.