Never, unless you have one and now two books that are so good they sell by themselves.
Promoting book one evolves into promoting the brand. And the brand is you. Part of every day must be dedicated to reaching out and communicating meaningfully to the people you are seeking to help, or entertain, or work with.
With every breath, you do your best to leave a trail of intellectual candy that taste so good, that after a few bites, people decide they need to have the whole bag.