Use the Columbia Acorn Fund as an example of a large holder. They have 50M shares (last known amount). With average daily volume of 1.2M it would take them about 42 trading days to close their position assuming no off-market transactions. Of course as you see, today was about 2.4M volume, so closure could be only 20-30 trading days with slightly accelerated volume.
The increase in volume over the daily average is one reason I wouldn't automatically assume this is small holders dumping in frustration, but rather a larger holder selling off quietly.
If you review the list of known large holders, only five would require significant time to close their position entirely. Many of the other Funds / Institutions are pretty small holdings.