In PopUp "Additinal Settings" of "Display Settings" section in Hustle, there's an "Expires" option to set. The value there only takes effect if either the "Add Never see this message again link" option is enabled and the user clicks that link in the popup or if "Close button acts as Never see this message again" option is enabled and user closed the popup by clicking on "Close" icon.
This might be confusing so it would be good if there was a line of explanation added in the plugin or the setting would only get activated if one of these options is set.
However, even if "Close button acts as Never see this message again" is enabled and "Clicking on the background does not close Pop-up" option is disabled, only clicking on "close button" sets a cookie so the user doesn't see the popup again. If the popup is closed by clicking on a background, the cookie is not set and the popup loads again.
In this case clicking on background should be equivalent of clicking on close button and should also act as "Never see this message again" or there should be an additional option to set enable/disable background to act as "never see this message again" as well.