my primary is basically a 20 gallon trash can made of food grade plastic. the top is not air tight. there is no water lock or anything, the CO2 just escapes through the gaps in the loose-fitting lid. how long do you think it is safe to leave my beer in there before racking? i know that the CO2 blanket should keep it good for a while, but is it safe for an extended primary fermentation of 3 to 4 weeks?