IP protection and Optimality

Comments on Shiro Armstrong "Intellectual property and wealth transfers in the Trans-Pacific Partnership", 31/03/2013, http://www.eastasiaforum.org/2013/03/31/intellectual-property-and-wealth-transfers-in-the-trans-pacific-partnership/
It may be the case that both theories and empirical studies can show pervasive and/or inconclusive results that would make the policy issue difficult to follow.
It is understandable that there needs some protection for IP because with no protection, there is little or no incentive for commercial research, innovation and invention which involves costs.
However, how much protection is needed to be optimal is anther issue.
It seems a time period is one of the ingredients of IP protection. But there should be other dimensions, such as a total amount although this is likely to be a case dependent, given the costs of each case are different.
The total amount or maximum amount should be linked to costs to allow a objectively set reasonable rents be extracted to maintain enough incentives but also allow other benefits to be realised.