A marriage certificate is issued after the marriage has been legalised. The marriage is registered with the relevant authorities and a certificate will be issued. It varies from place to place and country to country – some places issue certificates while you wait, and some will mail it to you after a few months.
The time that it takes to get a marriage certificate will largely depend on the country or state in which you live. If you are in the United States it is best to look in your phone book’s city pages under “Marriage” or search the internet for the requirements of getting a marriage certificate. If you are in India, the certificate can be issued after the marriage has been solemnized, and there is no rule as to how long it would take; sometimes it may take a few hours, and other times it may take one or two weeks to get the marriage certificate.