Hello pixelbotstudios,
As long as the interstitial is still in the cache, unless the cache is flushed (e.g. on app close or some other way), even after x amount of time, the show of the ad will count as an impression. If the ad can be viewed from the cache, the impression will be counted.
Each request will only count one valid impression for Marketplace ads if the ad shows.
I hope this clarifies the situation. Let us know if you have any further questions.