Average duration of a solar cycle is:

The solar cycle, commonly referred to as the sunspot cycle, is a nearly periodic change in the sun's activity and appearance. This cycle tracks the rise and fall of the number of sunspots visible on the solar surface. Centuries of scientific observation have shown that this cycle takes an average of about 11 years to complete. Over this 11-year span, sunspot numbers start at a minimum, gradually rise to a maximum peak (which vastly improves higher frequency HF propagation), and then slowly decay back to a minimum.