As a general guide:

5% is a hill (as opposed to a roller, or a slight upgrade)
10% is a steep hill. Most riders will drop down to their lowest gears.
15% is a really steep hill. Even with low gearing (say, 30-25), some/many riders will need to tack back and forth and/or stand
20% is super steep. Almost everybody will have to tack.

Some riders who would be riding their first century might need to walk on a 10%