A benchmark will be good for 3 years. If you conduct a Sustainment, then the maximum validity is 2 years, with a reduced scope, assuming scoping, etc. is the same (See MDD). This includes a maximum of 3 Sustainment appraisals, after which another Benchmark must be conducted to maintain the rating. A Sustainment appraisal cannot be used to raise the rating.
Keeping with CMMI V2.0 project goals, we have continually sought and responded to community and market feedback, and this topic is one where the community has been historically split.
So the solution is to allow both options, at the discretion of the sponsor, and then we will continue to monitor, measure, and analyze the data to determine if both the reliability and cost reduction objectives are being met.