Petra

A week or two is the usual guidance for concentrate, and the honest answer is make smaller batches rather than trying to find the edge. Two litres that you finish in five days never gets the chance to do this.

That is fermentation and the fizz settles it. Brewed coffee has dissolved sugars and other compounds that wild yeast and bacteria will happily work on given time and warmth, and you gave them both: eighteen hours on a warm counter and then four days in a jar.

Move the steep into the fridge. It takes a little longer and it is far less likely to go anywhere. Boozy, sour, fizzy or off, or anything with mould, film or cloudiness in it, pour away.