First Fishless cycle questions.

You can test it right away. Add enouugh to get you up to 5ppm. Then when you show nitrite decrease the drops to 2-3.

And by right away, give it enough time to mix so you're not testing the batch that you just added or a patch of water without any.

How to know? {tank volume/filter flow rate} should be the amount of time necessary to turn over the full tank volume. You may want to err a little on the liberal side and double this value to be certain that your tank water is fully mixed.