Martyn Emery provides a general rule of thumb.
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Whilst there can be no generic standards in terms of Y2K defect rates. The concept of assuring a high quality threshold is applicable when it comes to the selection of companies who are performing offshore remedial activities. If these companies can not demonstrate that they measure defects, how on earth can they be sure about the quality of their work?
A good figure for defects at present would be 5-10 defects per thousand lines of changed code. This is after acceptance test, and counting defects from December 99 through to April 2001.
So if you modify a million lines of code, plan for 5-10,000 defects.