Where is the expiration option?

Greetings,

I'd like my form available from 6 AM - Midnight only. Is this possible?

How do I set this up

Unfortunately, it is not possible to set an expiration option as you're describing, at least there is not a setting that will allow you to do that, you can only limit the form to an specific date or certain amount of submissions, here is a guide with more info about this: How-to-limit-submissions-in-a-form

I would suggest you to apply this workaround to achieve what you want, first add a time field to your form and set the time to the current one: 

Then you can handle this with conditions and show a message if the time is not the one you want, hide the fields in the form, the condition should be like this one: 

Here is a guide that will provide you with more info about how to set conditions based on another fields input: How-to-Show-or-Hide-Fields-Base-on-User-s-Answer

Please do let us know if you have questions, we will be glad to help.