Is it normal for iPhone to take longer than 8 hours to erase contents and settings? I've been waiting 8 hours now for my iPhone 5C to erase content and settings and still not finished.

How long is it normal to take to reset an iPhone to factory 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.