The solar activity cycle, when taking into account the polarity of the Sun's magnetic field, lasts approximately 22 years.

Here's why:

* The Sun's magnetic field flips every 11 years. This means the magnetic north and south poles switch places.

* The solar cycle is defined as the period between two successive maxima (peaks) of solar activity. These maxima occur when the Sun's magnetic field is at its strongest.

* Therefore, a complete cycle encompasses two 11-year periods. This is because the Sun's magnetic field has returned to its original orientation, including its polarity, after 22 years.

So, while we often talk about an 11-year solar cycle, when considering the magnetic field's complete reversal, the true cycle is 22 years long.