Google doesn’t offer a way to set the time when a Google Form closes for responses, but that’s not going to stop those of us that teacher like a hacker! I was trying to think of ways that you could close a Google Form, and realized that one way to do it is to use an url shortener to share the link to the form, and have the url shortener expire the shortlink after a set amount of time. This doesn’t stop someone from sharing the original link, but it works pretty well.
urlgone.com has an option to set the number of days before the shortlink expires. By copying the url of the Google Form (preview the form to get the address of the form) and pasting it into urlgone, you can set the shortlink to expire after a set number of days.