Different user bases will have different requirements - some will be much more likely to be able to respond to the registration email more swiftly and therefore the security can be improved by reducing the length of time before the email expires.

The option that generates the registration form complete with user details (which the user is emailed a link to) is called 'Use email links to autofill forms', under Email settings. Where it says 'auto-fill expires for the links after 30 days', we want this to be a feature that is configurable by NT/System admin.