Say you run out of impressions, if you wait like a week to refill them do you have to resubmit the billboard?

I have two that have been expired for a few weeks now (2-3 around) that are still there. Not entirely sure how long they stay there for, or if they even go away at some point.

I have a billboard that ran out about 2 months ago still in my list so they stay there for quite a while, so there's probably no rush to refill it until you're ready too.

Thank you =)