Precise cooking time: how to figure it out

It is very simple.

1. Note the time you start cooking it, and the flame/power setting/temperature.
2. Check periodically to see if food is cooked
3. When food is done: record time and subtract from start time.

If you cook the same dish a few times, you can average it out. A lot of cook times are approximate anyway; you have to check the dish a bit before it should be done to make sure it hasn't cooked faster than expected.

Cooking is not an "exact science" in my view. You're not always using the "exactly" same temperature, "exactly" the same amount of oil/water, or cooking a cauliflaur of "exactly" the same size as before.

I think that lots of it comes from experience, sometimes you just "know" that something is ready even though you cooked it 30 seconds less long than the last time you prepared it.

EDIT

Expanding on Jefromis comment, I didn't mean to dispute that, of course, you should have an idea of how long you approximately need to cook something. I usually use recipes off the internet or from one of our hundreds of cookery books as a starting point, whenever we try something new for the first time.