Forever.
Although the snapshot may be deleted if it violates the rules of the hosting provider (for example, if the page contains pornography or used as the landing page for spam campaigns).
I do not plan to shut down the site.
But I like your idea to have backup on a 3rd party public site in a form that anyone can download, say, a zip file with any snapshot. The only problem is: there are more than 10 terabytes of the data and growing quickly.
sure!