John Mueller of Google said in a post on Reddit that disallowing Googlebot to crawl your site would not immediately lead to the site being deindexed. He added that “it would take quite some time for the whole site to drop out,” and the home page may not be dropped at all.
Gary Illyes from Google said on LinkedIn that if your server returns a 500/503 HTTP status code for an extended period of time for your robots.txt file, then Google may remove your site completely from Google Search.
Google’s John Mueller repeated it, the time spent on a page by your users is not a factor Google uses for ranking. Time spent on the page is not a Google ranking factor, he said, “It doesn’t,” meaning Google doesn’t care about time spent on the page for ranking purposes.