The best answer I can give you is that you should share as much product knowledge as the customer whats and needs to hear. No more, no less. Of course, this requires you to be an expert on your product or service. Being an expert is one thing; being a bore is something else.
Just because you know it does not mean you have to get it all out at the point of sale. You only need to give customers what they need and want to know. Use your knowledge as a source of confidence, not a stick to hit them with to prove your worth!
I would add that you should take time to understand how your product or service is used by your market, not just what it does. Armed with this additional knowledge, you can be more appealing to your prospect and be a more effective adviser to them.