I'll chime in from recent experience. I had to pull a dependency in my project because it was causing heisenbugs that I couldn't root out. Without tests, I wouldn't have known. With tests, it took 2 hours to replace (diff -200 +250), and I'm confident the library still works. Without tests, it would have taken several more, at least.
I write tests as I develop, and they take the place of going to the browser or a repl and poking around at my work. In some sense, testing takes zero or negative net time. It would be hard to measure gross time, because it occurs in such small increments.
