Determining Active Dates For Queued Items?

I have a limited number of ad slots on my website, let's say there are
5. I have lots of people wanting to advertise in those slots, let's
say there are 100. They each join the queue for an ad slot on a first-
come, first-served basis.

I have been trying to find a way to
determine the active date for a queued item. For the first five items
in the queue it's easy, it's just the expiry date of the item in a
live slot that expires first. For the next five queued items it's
still easy, because it's just the expiry date of the item in the queue
that will expire first.

HOWEVER, after that it becomes more complex,
because the queued items will continually be slotting into live slots
as live items expire. So my question is, is there a straightforward
way, in ASP, of determining the active date for a queued item in a
queue of arbitrary size?