[Metric] Average expansion time [Description] 
For Rich Media Expanding creatives, the average amount of time, in seconds, that each expanded creative was displayed in its expanded state. Only one expansion is counted per impression. If a user expands a creative, closes it, and expands it again, the expansion time is cumulative. Once the creative has been displayed for more than 20 minutes, no further expansion time is counted. The expansion time does not take into account Rich Media impressions for which no expansion was recorded.