The amount of time for which an ad, dynamic contents, banners can be seen on a website is called viewtime.
Longer the viewtime more would be the cost of placing that ad banner on a website. Companies often buy viewtime on website for online marketing.
Sometimes a content with large viewtime might annoy the users.
Youtube host many videos and places a small ad before some videos. Companies have to buy that space from Youtube at a certain price based on the duration of ad.