A place to discuss software maintenance:

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Maintenance is the modification of a software product after delivery to correct faults, improve performance, or other product attributes, or to adapt the product to a new or changing environment.

In various studies, maintenance costs have been observed to consume from 50% to 80% of total life cycle costs, and typically consume 65% to 75% of total life cycle costs.

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Types of software maintenance:

Corrective Maintenance - diagnosis and correction of errors

Adaptive Maintenance - modifies software product to properly interface with a changing environment

Perfective Maintenance - enhancements to product to either add new capabilities or modify existing functions

Preventive Maintenance - changes made to improve future maintainability (reverse engineering/re-engineering)
Approximate proportions:

Corrective 21%

Adaptive 25%

Perfective 50%

Preventive 4%

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According to Larry English of Information Impact International, "70 percent of all computer printouts were used to re-enter data into other databases." "One company reported that 80 - 90 percent of developers’ time was devoted to ‘maintenance’ and of that ‘maintenance,’ 60 - 70 percent was devoted to maintaining interfaces, copying and transforming data from database to database." "Another company reported expending $100 million per year in patching programs and fixing errors in data, created when passing data from one system to another."

Source: http://www.zifa.com/zifajz01.htm

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See http://www2.umassd.edu/CoursePages/SoftwareEngineering/LectureMat/maintenance.html
http://www.cpsc.ucalgary.ca/~lamwa/SENG/621/SoftwareMaintenance.htm
For pragmatic views, http://www.cs.monash.edu.au/~damian/CSC2050/Topics/13.25.SWEng4/html/text.html#preventative_maintenance
A decent academic outline: http://www.cstp.umkc.edu/~hines/458/mdef.html
Why Is Software Maintenance So Expensive

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