The problem is that there can be a significant amount of time between wanting to protect an idea and when you can effectively monetize that idea. It might take an inventor 10 years to work out the kinks in an invention and build a market for it to the point that they can really reap the rewards. I think anywhere between 10 and 20 years is a reasonable range.

On the other hand, copyright terms are just bat-shit insane and needs to be scaled back tremendously.