Volumes can and have been written on this very subject.
It's going to vary between firms and between each project. You're going to want to ask yourself afew questions when planning your regression.
<LI>How critical is the system? (space shot would be pretty high, a game might not be.)
<LI>What section(s) might the new feature or change affect?
<LI>How much time, money, effort can this project afford?
<LI>Do you have a automated tool for the regression testing?
Additionally, when planning the first set of test scripts/cases, you'll have a bigger picture in front of you, so you might want to mark certain features or test paths at that time that you will always want to run during regression testing.
Plus, you've probably got a rating from the business on the criticallity of each of the features, you'll want to do a little regression on those.
Some of your original test cases might point to some critical areas or fragile areas in your system. Add that to your regression bed as well.
Please don't take my suggestions as a complete guide to regression testing. Use the search tool in your upper right corner and look for posts on this subject.
Check out the links tab at the top of this page, you'll find there's a host of books and papers that you can get information from.
And I'll bet one or two of the guru's here will stop by and add to the list of things to consider when planning your regression testing.
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Indeed, it is the only thing that ever has." Margaret Mead