There is some possibility of being sued. Even if you are not charging them for anything, if you are getting any economic value from them using your site, that might be sufficient "consideration" (exchange of value) to support a court finding that there is a contract. If there is written agreement, the court might then enforce the terms of that agreement, as to for how long you'd keep the site up, or the notice required before taking it down. Even if there is no written contract, if there was an oral agreement or you'd otherwise made oral promises that you would provide a certain amount of notice before taking it down or would not take it down without notice, a court might enforce that.So if you have made any promises or representations, either in writing or orally, about not taking the site down, having it up for a certain periord or duration, or about the notice you'd provide before taking it down, it is possible that they could sue to enforce those representations.