How long can I store the wort in the no chill cube? Does it need to be used as soon as its cooled down or relatively quickly?
It comes down to your sanitation and process. You want to be filling those cubes hot. This means above 190ºF. This will, to a significant degree, sanitize the inside of the container and ensure that you'll be able to store it for a reasonable period of time. Many no chill brewers go for weeks or even months before pitching yeast.