One of the most important uses of tests is that developers run them while they are editing code, to see if the new code they've written is actually working. As tests become slower, they become less and less useful for this purpose. Or developers continue to use them but start writing code more and more slowly because they keep having to wait for the tests to finish.
In general, a test suite should not take so long that a developer becomes distracted from their work and loses focus while they wait for it to complete. Existing research indicates this takes somewhere between 2 and 30 seconds for most developers. Thus, a test suite used by developers during code editing should take roughly that length of time to run. It might be okay for it to take a few minutes, but that wouldn't be ideal. It would definitely not be okay for it to take ten minutes, under most circumstances.
There are other reasons to have fast tests beyond just the developer's code editing cycle. At the extreme, slow tests...