Creator Insider channel who are the ones who work FOR YouTube put out a video that goes into more detail about how Recommended works
View: https://youtu.be/9Fn79qJa2Fc
Essentially long story short is there's no specific amount of time it takes for youtube to recommend videos. Some get recommended instantly, I had one video that took 3 months to get recommended.
Recommended revolves around more than just audience retention, it's about audience ENJOYMENT, which they say goes beyond just views. Naturally, if a video is watched longer, and a watch session is completed more, that gives YouTube signals that the video is more enjoyable, and is more likely to be recommended. They actually use likes and dislikes as a factor for determining enjoyment, I always thought dislikes didn't matter, but apparently they do add a factor. Engagement seems to be key, how many comments a video gets, the likes, the dislikes, the overall audience completion rate, how many other videos of yours the audience watches BECAUSE of that video. There's too many factors to put an exact definition of how long it takes, if ever.
You always help your chances with consistently good content that engages, and pleases your audience.
I have a bunch of videos that have very low audience retention, and when I look into my impressions, it's a sad reality that if your video has low audience retention, the suggested impressions will suffer as a result. And high search impressions doesn't always mean high suggested impressions. If your video is on a topic of interest, search can help boost suggested as well, but ultimately search helps people get to your video, and it's up to the quality of your video it'self (value of entertainment) that decides if it goes anywhere beyond search.
I've been heavily investigating this myself for my own channel as my recommended traffic dropped significantly in 2018 when the algorthm followed human behaviour more. Thanks to TubeBuddy I've learned how to read my data, and to understand what that data means, so I can make informed choices on where to take my future content which will ultimately improve my recommended impressions long term as long as I keep working hard on quality and studying and constantly learning and evolving