How long does DNS Validation takes

Are you experiencing problems with DNS validation, or just asking out of curiosity? It usually doesn't take long, but "Your new certificate might continue to display a status of Pending validation for up to 30 minutes." (https://docs.aws.amazon.com/acm/latest/userguide/dns-validation.html) Also note that if it hasn't been validated within 72 hours, it will get status "Validation time out".

This is what I was missing. Once you submitted the request to create the Certificate, you must perform the "Setting up DNS validation" in the above wiki.