I'm implementing Varnish in one of our projects and I want a long TTL on the HTML generated by Concrete5. For this reason I'm implementing edge side includes for some parts of the website. The form key is one of these parts that I want to load using ESI(edge side includes). This seems to work although there is some room for improvement.

Basically, I was wondering how long a form key is valid(So I can let Varnish cache this for a short period). If this form key is only valid for one person(using cookies). If the form key can be used more than once within the time period in which is valid.