From your Admin portal, click on Forms, then select the form you want to set a frequency for.

You'll see the Assignments & Frequency tab at the top. 

This tab should be populated with a list of your assets at the top and locations at the bottom and you can make changes to the frequency here:

Any Asset frequency set as N/A will not be available for workers to submit this form to. Optional is just how it sounds, they can if they like, but there is no requirement to do so. When you select Required, you can set a frequency in which workers must submit a form for this item.