As for March 1, 2018 a new requirement of the CA/B Forum will be applied: the maximum validity period of SSL-certificates will be limited to 24 months.

What's CA/B Forum?
CA/B Forum is an industry regulation body that creates and adopts rules related to the procedures of issuance and verification of SSL certificates.

New maximum validity terms for SSL certificates
As for Ballot 193 proposal, there's a new limit on the maximum period of validity of all trusted SSL certificates - 825 days. This would be a two years period with additional days provided for a renewal and replacing an expired certificate. This new amendment has been finally approved and will be applied beginning from March 1, 2018.

Currently, the maximum validity period for an SSL certificate is 3 years (in regards to SSL certificates 3 years means up to 39 months).
Certification authorities can issue DV and OV SSL certificates for a period of three years until March 1, 2018.
Since March 1 the issuance of SSL-certificates will be possible only for 1 or 2 years.

Why validity period of SSL-certificates is reducing?
SSL Certificates users are more confortable with long-term certificates (many would be happy with a 5-years ccertificate, or even never-ending...), so there would be no issue nor time wasting in reinstalling them.

But SSL industry has a different point of view, and different goals; and making a user regularly update his own certificate is a security measure.

In the future, the industry will continue to decrease the validity period of SSL certificates. Although these changes will be implemented slowly, all users should be prepared for regular updates (about once a year) of their certificates.