The longest period for validation is 3 years, which has discount vary from 10 to 20 percent. The 3-year limit is decided by two factors: One is a requirement by the standard, that the longest period for a certificate must be no more than 39 months. The other is that, in too long a period, identity status of an organization or individual may change.
As for the free encrypting certificate, MeSince will issue a new one every 39 months. For identity certificate, before it expires, please re-apply the validation in your account and MeSince will issue a new one.
Certainly, you can pay fees for more than 3 years of validation fee and after 3 years, you only need to do a new validation process.