String theory - Knot a clue

Just how long is a piece of string? I was genuinely asking myself that question today as I was getting together more bits of kit. Specifically, how much to take for a drying line along with any pannier/tent emergencies? Well I settled for about 12 feet ... or should I take more? ... or is that too much? ... is it the right type of string? ........ arrgghh!