I have hundreds of tests that take 15-30 mintues each. These tests tend to be whole system tests so there is no way useful way to say it won't touch your change (75% will). Despite an extensive unit test suite (that runs first) these tests catch a large number of real production bugs, and most of them are things that a quicker running test couldn't catch.

Which is to say that trying to avoid running tests isn't the right answer. Make them as fast as you can, but be prepared to pay the price - either a lot of parrell build systems, or lower quality.