SSL certificates are issued within 24 hours of successfully connecting your domain. If 24 hours pass and the SSL certificate hasn't applied, then you'll want to check your domain to ensure it does not have additional "A" records or "AAAA" records as these records will cause issues with the SSL being applied. You can confirm if your domain has additional "A" or "AAAA" records by using free tools such as MXtoolbox. Removing these records if they appear should resolve the issue within 24 hours.
If your domain does not have any additional records and the SSL hasn't applied after 24 hours then you'll need to contact Shopify Support so they can reprovision the certificates. You can do this by logging into your store through the help center, searching your question/issue, then selecting "continue" at the bottom of the page that displays the search results.