Some time ago I ran an analysis based on feedback from several video ad networks and it turned out that eCPM goes down a lot after the first 4 impressions.
Looking at the trend, if the eCPM of the first impression is 100%, you can see how much will it decrease when the same user watches several videos:
From the advertiser perspective, the more videos the same user watches, the lower her/his conversion rate is (they will be less likely to click on an ad and actually install the advertised app).
My advise in general would be to focus on the number of users who watch videos (so called engagement) rather than on the number of videos watched by the same users ( known as frequency).
