The present leak was discovered on 20 December 2011, and came from a break in a flexible line about 360 meters out from the vessel that sends oil to tankers. At that time it had probably spewed for hours before being noticed.
Pollution from spilled oil stains Nigeria’s Niger Delta region, with crude oil lapping against beaches and leaving a black ring around creeks in an area about the size of Portugal. Possibly as much as 550 million gallons of oil poured into the delta during Shell's roughly 50 years of production in Nigeria a rate roughly comparable to one Exxon Valdez disaster per year.
PS of 28 December 2011:
An environmental group said yesterday that an oil slick had approached Nigeria's coastline after the Shell spill last week - although Shell insisted that its spill had been largely dispersed. See http://www.physorg.com/news/2011-12-oil-nigerian-coast-major-shell.html