My experience has been to brew 18-24 hours in a refrigerator. Brewing at a lower temperature is nice because it reduces the amount of oxidation that occurs before you can even drink it, but because you are brewing at a lower temp, it takes longer to reach full brew strength (just think about it: faster, more energetic water molecules will extract water-soluble compounds more quickly than slower methods). Either way, it really pays off to have an airtight container. Hope this helps!