If The Standing Orders Act applies to your organisation (check) the probation period prescribed is 3 months. If not the industry practice is 6 month's probation period which can be extended at the discretion of the management.
Usually the probation period is applicable for permanent job and meant to assess. each other's suitability.
The services ,during probation period can be terminated by either party without notice or assigning any reason.
It seems from your profile that you are very senior professional and definitely the members of this forum will take something from you.
With due respect to you Sir, I would like to know from you who are the employees not covered in the definition of workmen under labour laws?