Original command which you might be using for generating the certificate:
openssl x509 -req
-in domain.com.csr
-signkey domain.com.key
-out domain.com.pem -extfile domain.com.ext
You need to add the -days argument to the above command following the days you want for your certificate expiry. The command will look like this:
openssl x509 -req
-in domain.com.csr
-signkey domain.com.key
-out domain.com.pem -extfile domain.com.ext
-days 120
Above 120 days is the expiry time for our self-signed certificate.