Here's the link to the section you needed in the training portal: https://dovetailmktg.kartra.com/portal/mgsitlogin/post/248
There is no "golden rule" or perfect timing for when to use retargeting. Ultimately you will have to test it to make sure it's performing well enough. You may need to try it out, and if it's not performing well, turn it off and wait for more time to pass before you turn it on again. Usually if it's not performing well, it's because enough people haven't visited your landing pages and/or engaged with your posts or page.
I usually won't even test running a retargeting ad to landing page visitors (that did not become a lead) until there has been at least 300 landing page visitors. Often times, even that is not enough. You will just have to test.