The Beer Advocate has an interesting forum discussion on this topic. The bottom line for a ceramic topped flip lid growler...
Sweetwater leaves about an inch of air in the neck of their growlers. Less would be better. But in my experience, you may keep these in the beer fridge for up to a month without any degradation.
Once open though, plan to drink it all within about 24 hours. For those of you that sip your beer, that means you can drink about half the first night, and finish it off the second night. Or share one with a buddy during an evening of watching March Madness.