The video that Oskar Blues did on Ball's website says it can stay good 3-4 weeks...if done properly, of course. If I recall my last place gave them a shelf life of 10 days, though we didn't have the machine set up and running until shortly after I moved out here. Of course, all this is with CO2 purge and I still wouldn't want to sit on it for much more than a week anyway. Regardless, they hadn't really caught on in Boston when I left so my experience is limited, so I'll defer to the wisdom of the crowd on this one.
Everywhere around here does crowlers and they all say drink in about a week.
Also, some are good for quite a while, some not so much.
And OB is constantly pushing the this isn't a can motto. Drink soon.