What is the “industry standard” ratio of new development to maintenance?

I am an application developer for a regional insurance provider. I work on “new projects” but also work with the maintenance team who maintains and enhances all legacy applications. One frustration that the business units, and some of the developers, have is that management and the pm’s believe that maintenance/enhancement items should make up only 20% of development time. The list of maintenance items and new enhancements to existing systems continues to grow. What is the “industry standard” ratio of maintenance to new development?