A lot depends on the size of the project. For a good understanding on the full process of regression testing, check out this link.
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Regression Testing Effort is usually included in the overall estimates of the project. The time is not fixed due to its critical environment. Proper Planning may help to reduce the time while it is needed for regressing testing the application. Thus depends on, the number of Critical Defects Uncovered during the Integration/User Acceptance Testing Phase. Or complexity of the change in the application. By a thumb rule, Regression Testing might not always default to 10% of the overall Testing Effort in any given project.