Does ProSites Delay Billing if a site already has status?

So this is more of a theoretical question.

If I grant a site Pro Site status for let's say 2 weeks - and then I want the user to set up thier subscription. If I send them to the page to modify their status and they sign up - will they be billed instantly - or when their current status expires.

In addition, will the billed amount account for the partial dues since they currently have status?