I deleted my old site entirely and made a new site 8 months ago. But still in the crawl errors section of Google Search Console, a lot of 404 and not found errors appear. All the URLs are for the old site. How long does it take for Google to stop giving these errors? What can I do in the meantime? 
Google crawl errors after deleting old site
seo;google search console;404
how long does it take for google to stop giving these errors?There is really no fixed time. For some of the more 'active' websites, I have noticed Google to make alterations to search index at a fast pace as compared to less visited (by Google bot). As correctly pointed out in comment section, wait it out.what can i do meanwhile?If all the urls being shown as 404 are indeed not available then skip the following suggestion. If not, try an redirect old url to new ones, this will help you keep traffic. Additionally, you can keep a check on indexed links and see if Google is removing them (or not).P.S - If you have a relatively young website, this can take some time.