How long the approval process takes. We already sent a test email to schema.whitelisting+sample@gmail.com and filled out the registration. How long the application process typically takes before we should expect to hear something back.
You can check my answer here: No response after filling in the Email Markup registration
As per my experience in requesting to whitelist a specific email address, usually, it takes about 2 weeks in total to complete the process. Just make sure that the following guidelines in this documentation was met to avoid rejection.