The daily roll yield since July 2010 has averaged negative 12%. This is the lowest 6 month average roll yield since VIX futures began trading in 2004. For perspective, the average roll yield over the entire graph is negative 5%.
With a single exception on November 6, 2010 of negative 2.5%, the daily roll yield has not breached negative 6.5% since July 6, 2010.
An average of ~5% of the contracts in VXX must be rolled daily (100% of contracts / 22 trading days per month). VXX provides current contract concentrations updated nightly. A long-term investor's return in VXX is largely determined by the average daily front month roll yield during the investor's holding period.
Disclaimer: Dataset created via Excel manual process
Source: CBOE settle prices
Note that December 2004, and April, July and September 2005 did not have contracts. In these cases, I graphed the difference between the two front-most contracts. Some of the noise in 2004 and 2005 can be attributed to this.
Roll Yield = (Front Month - Second Month)/Front Month * 100
Disclosure(s): Short VXX at time of writing