Please, explain a little better.
Do you have a payment plugin, or is something like sandbox paypal without any plugin installed?
With a payment plugin you already have the option to choose how many days to premium listing, or to create new packs with premium days, or something similar to do it. After terminus of premium date, the listing will be demoted to normal listing until normal expiration (30 days in your case/configuration). That means, the premium listing will be public during 3 days + 27 days as normal listing = 30 days on total.