Is there out there some work about software maintenance as a function of application size measured in Function Points? Something like an average of 33% of the application size is delivered as enhancement projects along the year. If the data is segmented by application age, even better.
I don’t know whether I understand your questions correctly.
Software Maintenance for me is removing defects. Enhancements is changed / added requirements.
Software Maintenance can be based on a certain percentage of new build effort on a yearly basis. Like 15 or 25% of the new build effort. If you have a higher percentage, than you can include some small enhancements (less than 2 weeks time & not involving db changes), but usually the larger enhancements are put into development projects.
There is no such thing as a certain percentage of the application should have enhancements! This really depends on how the business is changing and can vary per business/customer.