A monofilament line does, in fact, break down and become sour after a period of time. To begin, the line is divided for UV light. To avoid UV separation, monofilament line should be stored in a dark compartment if at all possible. Second, a monofilament line splits over time; monofilament typically has a shelflife of 2-3 years. When properly stored, it has the potential to last longer.