Do you have plastic or metal caps for your growler?
Once you open it you begin to lose carbonation. After about 48 hours from first opening it, my beer is usually completely flat. If you can't drink a 64oz growler in 48 hours, though, that's likely a problem in itself. If you leave it sealed and then kill it all at once, I'd imagine you can get 4 or 5 days max before you'd need to go ahead and consume it.