Once the temperature gets above 80 and we start swimming every day, My 5 gallon barrel lasts 7 or 8 days. When I change it, it usually has about a gallon left in it, which I use to supplement in the evening. I usually keep a routine of changing the barrel on Sunday evening, So I can remember before it empties out. If it does empty, it just adds air to the line until I get to it.