Google Forms does not have a native countdown timer. There is no setting for it, and there is no per-question time limit. If you need a quiz to close after 30 minutes, you have to add the clock yourself.
Add-ons such as Form Timer or Form Limiter can close a form at a set time. They run in the browser, so you need a laptop to set them up, and the timer applies to the form as a whole rather than to an individual respondent.
A timed link gives each respondent their own countdown, starting when they open the form rather than at a fixed wall-clock time. That is usually what a classroom quiz actually needs: every student gets the same 30 minutes, whenever they begin.
A per-respondent countdown and a fixed closing time solve different problems. Use a schedule when the deadline is the same for everyone — end of the lesson, end of the day. Use a timer when each person should get the same amount of time.
Google Forms is built as a general-purpose survey tool rather than an exam tool. Timing, response caps and per-option limits are all left to add-ons, which is why every answer to this question involves adding something.
AI Forms for Google Docs does this from your phone — no add-on, no laptop. Get it on the App Store.