After the time period the patent / copy rights should be released to the public domain so that everyone may benefit from cheap economy of scale production.
The only reason I could think you feel this way is that it is all you've known. In most cases a couple of years worth of profits from monopoly should suffice. In reality it depends on how much was spent on the initial innovation.
One tricky issue is also the lead time to market. In the synthesis example, Yamaha came out with a synth within ~2 years of licensing the patent. But in some areas (like medicine) it can be common for it to take 5-10 years for a product to come out, in which case a 5-year patent would be worthless, because it would expire before you can sell anything.
In reality it depends on how much was spent on the initial innovation [and how long it takes to reap the return on that investment].
It seems to me that you want it to be less for your convenience, and no other factor.