This add-on will stop your Form from accepting new submissions at a certain date / time, and/or a certain number of submissions, with some limitations. 

LIMITATIONS:

Google Forms can not display a count down timer or a custom message to the user about how much time is left before the Form will stop accepting responses.  Also, this add-on depends on time based triggers, and time based triggers are not guaranteed to run at the exact minute of the setting.

This add-on is not good for very precise timing, like a student taking a 50 minute test.

What it can do:

It can stop your Form from accepting new submissions at a certain number of submissions, and/or at a certain date and time.  This Add-on can use both date/time and number limits, or just the date limit, or just the number limit.

It can also make the Form start accepting Form responses at a certain date and time.  If you want the Form to start accepting submissions immediately, then check the box to "Start Immediately." The stop limit can be either a stop date or a certain number of submissions, or both. For example, you can have the Form stop accepting responses when either a certain date occurs or a certain number of Form responses happens. If both a stop date and a stop number limit is set, the Form stops accepting responses when whichever of the two criteria are met first.

The Start, Stop and Time Zone sections are outlined and highlighted with different background colors.

This add-on uses triggers. Triggers cause code to run when an event happens. Two different types of triggers are used. The two trigger types are:

On Form Submit - Runs whenever there is a Form Submission
Time - Runs at a set time
The "On Form Submit" trigger is used to monitor the number of submissions that were made. The Time trigger is used to monitor the date and time.