You should test all of it all of the time. Revisions, enhancements, and new development should all be treated the same: 100% coverage where possible. You see, revisions and enhancements can break existing code that worked - so they should be represented by test scenarios that are added to an existing body of scenarios. All scenarios are then tested.

>The assertion has been made that typical applications have between 40%-50% of their code actually run during testing.
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>I am curious to see what percentage of code you test (actually run during development/unit test/system testing) for:
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>1) The development of a system from scratch.
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>2) The development of a new/replacement system but using code parts (including classes, functions, etc.) from existing and well-running systems.
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>3) Developing revisions to an existing application.
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>Thanks for your reply.