How fast do I need to finish a Hefe Weissbier Keg? If I remember correctly, the website of Erdinger states their 'Dunkle Hefe' needs to be finished within just 3 days after opening. I know it isn't pasteurized but I wonder, can we stretch that a little? 3 days for a keg is kind of short....
Why is that anyways? Is it because the yeast settles?
Where did you read this? I am unfamiliar with Dunkle Hefe exactly, but this is true for wooden casks. 
Typically a wooden cask is served with gravity or with a hand pump to lift from a cellar. There is an opening in the top that draws air in to equalise the pressure. This air will reduce the shelf life to 3 days.
A "Donkey Pump" on the common sankey keg is similar. The beer will spoil quite fast.