A YouTube video typically has 50% of its views in the first 6 days, and 75% in the first 20

"A video on YouTube gets 50% of its views in the first 6 days it is on the site, according to data from analytics firm TubeMogul. After 20 days, a YouTube video has had 75% of its total views.
That's a really short life span for YouTube videos, and it's probably getting shorter. In 2008, it took 14 days for a video to get 50% of its views and 44 days to get 75% of its views."
Source: Data from TubeMogul, reported by Business Insider, 27th May 2010