Hello Musabani,
You could have given the details about the actual issue you are facing, rather than ask a generic query. However, there is no hard and fast rule about the duration of the probation period—in fact, some companies have a one-year period. The reason for having a probation period in place is twofold. Firstly, to give the new employee the chance to evaluate/judge whether he/she can fit into the job role for which he/she has been hired. Secondly, to give the company the chance to evaluate/judge whether the new employee really fits into the job role for which he/she has been hired. The basic premise for this is the realization that an interview conducted for a few hours or at most for a few days may be too little for each of them to make a well-informed and accurate judgment about the job/employee.
Hope this answers your query. If you need more details, please mention the actual situation.
Regards,
TS