How long should it for published changed to reflect on the front end?

It should be no more than 5 minutes for all customers.

It's based on when the config cache expires which is every five minutes, so if the merchant visits the site at 10:01am their config will expire at 10:06am so any changes made between 10:01am and 10:06am will appear after 10:06am.

Also, if the merchant make the change at 10:03am, even though they won't see it until 10:06am other customers may begin seeing it immediately depending on when their 5 minute cache is set to expire.