How long should the user wait before providing reassurance?

I have an application with user login. When a user logs in for the first time, I show them a screen similar to this while the login process completes:

The user's access must be validated before they can access any part of the app so it makes sense to block their access while we wait.

The problem is that, due to reasons out of my control, authentication can take anywhere between 3 seconds and 1 minute, because of the backend identity provider. So this icon is on the screen for a long time.

During testing, I've noticed that users start to feel like something is broken and want to reload/leave the page after a bit of waiting when authentication is slow.

I want to supplement the "Logging you in..." text after the user has been waiting for a while by adding in "...this can take a while. Please do not navigate away or reload the page."

How long should I wait to present this additional "reassurance" to the user? Also, is there a better way to comfort the user that everything is working, just taking a while?

Note: I did see this question, which answers a related question of "how long should I wait to show the initial loading indicator to a user?" - not "how long should I wait to supplement the initial loading indicator?" - Loading time and page loader display (when to show)