Unit test execution speed (how many tests per second?)

Question

What kind of execution rate do you aim for with your unit tests (# test per second)? How long is too long for an individual unit test?

I\'d be interested in knowing if

Answer 1

Some frameworks provide automatic execution of specific unit tests based on heuristics such as last-modified time. For Ruby and Rails, AutoTest provides much faster and responsive execution of the tests -- when I save a Rails model app/models/foo.rb, the corresponding unit tests in test/unit/foo_test.rb get run.

I don't know if anything similar exists for other platforms, but it would make sense.