One of the most important rules about unit tests is they should run fast.
Developers should be able to run the whole suite of unit tests in seconds, and definitely not in minutes and minutes. Developers should be able to quickly run them after changing the code in anyway. If it takes too long, they won't bother running them and you lose one of the main benefits of the tests. We currently have about 800 tests that run in around 30 seconds.
So if any tests do start taking too long, you can see which test is taking too long and do something about it.
Our unit test framework tells us how long it took for each test to run. Most of them take 0- 5ms (if I remember correctly), we have one or two that take about 3 seconds.