Not an easy one to define. As mentioned above, in a bare-knuckle fight, all it needs is a lucky punch.
Hard is much more calcuable back in school days, where a scrap had the ability to last several minutes. It is the last time most people get in to a real fight, and they fail to realise how things change as you get older/bigger/stronger.
Usually, with two average adult males, the winner of the fight is the one who lands the first good punch at their opponents head. The 'Hard' people, and experienced pub scrappers etc, usually appreciate this and simply hit first.
And from that comes my advice (if walking away is not an option): If your likely to get hit, hit first. You will be surprised what an average adult male can do without gloves.