How long may Unit tests take in TDD?

I have a general question about TDD.

As we all know, Test Driven Development requires a lot of testing. In best practice, you should test your code after every save. Therefor it is very reasonable, to keep your tests as fast as possible. The question is now: How slow is the slowest possible. Do we start in the minutes area? Seconds? What is best? As Example, I have a test, which has a 3x3 Testmatrix. Executing this test takes a few seconds. Assuming, this will sum up, it could by one day take a few minutes, to test a package. This would mean, that a programmer would waste up to an hour each day, waiting.

So the Question is: What is the maximum time, a test may take?