First, this really has nothing to do with ConfigMgr -- ConfigMgr is simply a consumer.
> "Does the PKI certificate for SCCM 2012 client certificate on the client automatically renew when it wil expire ? "
Assuming you've enabled auto-renewal and the client system has access to AD and the issuing CA, then yes.
> "What is the time that the certificate will be renewed automatically?"
Based on the text of the option in Group Policy, the certificate will be renewed once it has expired: "Renew expired certificates, update pending certificates, and remove revoked certificates"
> "Do we need to renew the template on the certificate server? "
Templates don't expire.
"Can someone explain how this works? "
How what works? Have you reviewed the official documentation?
Jason | http://blog.configmgrftw.com | @jasonsandys