I believe a generic criteria might not help you to decide how much regression you shall do. When you say full regression than i am assuming that you already have a regression test suite in place.
In practice how much regression needs to be done depends on the complexity/amount and impact the changes done to the code in the upcoming release are done.Your regression test suite shall cover all those impacted areas. Ideally you are the only person who can know what is impacted what not based on your discussions with development team and other stakeholders.
So in summary, any test coverage which fully cover the impacted areas(within and outside the AUT) is the full regression done. Your existing regression test suite shall evolve with the time and include the new error prone areas and exclude the stable areas.
I have been doing this a long time and I believe regression should always be done. Maybe not on a big scale because that is sometimes hard to do, but every test case you have, work regression into it for the areas that might be affected by the bug or enhancement being tested. Start forming a regression test case. What I would do was once I had the test case, I would have newbies I just hired go through it. Mainly before we were ready to release. They would get to know the system this way and it was also a fresh pair of eyes on the document to ensure nothing was missing, it made sense, etc.
Speak to development team leads and understand the impacted areas of the application and other applications interaction. Document it formally and then Look back to your old releases test cases and pick those important test cases that cover full functionality / main functioanlity of that particular impacted module/area of your application.
Your decision will also depend on how much you plan to spend time in regression testing each time you are doing a testing cycle.
How many resources will be involved in testing.
What if you fail to meet the deadline of testing for new changes and bug fixes which will hamper regression testing timeslines.
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