Regarding Superannuation

what should be tenure period should employee work to get superannuation benefits in any organisation.

Gratuity is provided as per payment of gratuity act. For that you need to follow the rules under the act and pay as provided.
Other superannuation benefits are voluntary. It depends on what the company wants to offer.
You can design any scheme You want, but remember that it needs to be fair and attractive enough to workers to be an incentive. If the period is too long, workers will simply ignore the scheme and it will not achieve your intent to retain talent
For example if you say superannuation will be after 20 years, which employee will bother ? Do any of them think they will work for 20 years for you ?

Superannuation means the age at which an employee retires from service and the benefit he gets at the time of retirement is Superannuation benefit. There are two superannuation benefits statutorily. They are 1) Provident Fund and Pension 2) Gratuity. One more, Superannuation Scheme through LIC or other Insurance Schemes are at the discretion of Managements. It is unfortunate that now a days PF/Pension and Gratuity are not used as Superannuation Benefits - for employees withdraw once they leave the employment. - G.SANKARANARAYANAN, HR CONSULTANT