Is it normal for iPhone to take longer than 8 hours to erase contents and settings?
Try again. This should not take more than a second. Everything on your iPhone is encrypted with a randomly-chosen encryption key, and all that's needed to "erase content and settings" is to discard that key. For security reasons, wiping the phone is designed to be nearly instantaneous so a thief has no time to intervene.

You might have better luck if you turn the phone off and on again before trying again. By "off" I don't mean screen sleep. Hold the power button down for several seconds until a "Swipe to turn off" slider appears.