I always test till I find at least one bug. This approach has never failed me, and I usually find more than one. Sometimes its a corner case but still a condition the software should handle. I would really have to write a very tiny thing in order not to make a single bug!

Of course archiving the tests can be a pain if nothing is prepared for it. Automating the tests is yet another pain. Still I worked on a library once when I added quite a few automated tests, because the infra was basically there and just waiting for it.