Well I guess those definitions really differ from name to name. But generally,
If you have no offers month after month your name is probably not great. It doesn't mean it's bad (but it could mean that it is), but just not premium etc.
If you are getting 1 to 5 offers a month. I think you have a good name that people are interested in.
If you are getting 5-20+ offers a month your domain is probably excellent and is in high demand.
It is important to remember though that it honestly doesn't matter how many offers you get if they are all lowball offers.
You are really intereted in how many offers are serious legitimate offers or are from people and companies that are truly looking to own the name (where you can negotiate and make a sale).
Skinny