With careful planning and initial human upkeep for the first few years, it's been shown this can be accelerated and have a self-sustaining forest in ~10 years. Akira Miyawaki (29.01.1928 - 07.16.2021) [0], a botanist who studyied plant ecology, seeds, and natural forest restoration methods, introduced a "reforestation" algorithm known as the "Miyawaki method" in the early 1970s [1]. It has been adopted throughout the world with great success. And also prompted a viral TED talk by Shubhendu Sharma [2] who then produced a 12 video series on YT on how to create your own forest [3].

[0] https://en.wikipedia.org/wiki/Akira_Miyawaki [1] https://www.af-info.or.jp/en/blueplanet/assets/pdf/list/2006... [2] https://youtu.be/mjUsobGWhs8 [3] https://youtube.com/playlist?list=PLDw6OmGaV5rnOCATIho19Icvp...