Since you don't want to distract your website visitors from the content of their website, you don't want to trigger a website survey too early on.
A good practice is to make the survey appear once the visitors have spent some time on your site. As a rule of thumb, you can ask for feedback once a visitor has spent min. 30 secs. on the page or has scrolled down halfway through the page. But, everything depends on your site's content and how users behave on it.