Switch from Wi-Fi to cellular (without Wi-Fi Assist being turned on)

I was connected to a WiFi network that had lost it's internet connection. My app had a message pop up "The Wi-Fi network does not appear to be connected to the internet - Do you want to temporarily use cellular data? - Choices - Use Cellular Data - Keep Trying WiFi" This is apparently a new "feature" in IOS 13 but I can't find any documentation on it. How long does it take to put up the message - how long does the bypass work?