Stop hitting me so hard!

Contact sparring, how much is too much? There are a variety of factors in making a proper judgement.

First of all, we have to define our terms - I will use the terms, "Light, Medium, and Heavy." I define them this way:

Light contact should be a touch, and nothing more. It should be heavy enough to let your partner know he was hit, but definitely would do no damage.
Medium contact is hard enough to "freeze" an opponent, and set up another technique. You will feel it, but it will have no lasting damage, nor will it cause you to lose consciousness, or "see stars."
Heavy contact is thrown with the intent to do damage, and will definitely stun or knock out an opponent if thrown with correct timing and distance.

To determine how much is too much, ask yourself:
What is the purpose of the sparring session or drill?
What kind of protection is being worn?

The purpose of the drill or sparring session determines the appropriate level of contact. If I am teaching a beginner how to block a haymaker, I will destroy her confidence by throwning it with knockout force and intent. In that case, I should use no contact, then light, then, at a high level of confidence (for her), medium. When she has experience with sparring and hard drills, then progress to heavy contact for the purpose of a pre-arranged drill only, and with complete foreknowledge. Build up, don't destroy your students. Conversely, with a trained contact fighter, light contact may handicap or destroy his ability to deal with what he will face in the competitive arena or on the street - a full contact, MMA or kickboxing fighter should probably vary between medium and heavy contact, depending on the goal of the session.

Finally, the protective gear worn can make a big difference in what level of contact to use. When someone is wearing a complete RedMan outfit, or similar gear, the purpose of the drill and the gear makes heavy contact the desired level.

Last, two warnings: Head contact is always potentially dangerous. Even with head protection and 16 oz. gloves, the brain still sloshes around, and is bruised by a heavy blow. I never recommend heavier than medium contact for sparring, only in drills or competition. Second, the headgear with a face mask is bogus! I have sparred with people wearing those, and all it does is extend the face out 3-4 inches and make it easier to hit them. I don't recommend it at all.