The period would vary from 2 years till 10 years, depending on your business nature / product nature / service area and the critical factors involved with it.
However, the basic issue would be how you or HR is going to monitor it or track it. Any employee can reveal the information once he is out from the company or relieved from company services.
With best regards,
The confidentiality clause will get terminated along with the termination of employment contract and hence it is very important to mention that the confidentiality clause will survive termination of employment contract.
Further, you can cause the employees moving out the company to again enter into a confidentiality agreement while making it part of the exit process of the company which would be applicable to any and all employees across the company.
Appointment letter is contract between employer and employee. This relationship ceases when the contract ceases to exist. Keeping this legal angle apart, what is important is to understand how to enforce the terms and conditions of the contract. Once the employee leaves the job, how will you prove or even find out that he/she has violated the clause of confidentiality?
Therefore, I recommend not to go too far in designing the terms of appointment.