  Duration of the extended solar cycles is taken into the consideration. The
beginning of cycles is counted from the moment of polarity reversal of
large-scale magnetic field in high latitudes, occurring in the sunspot cycle n
till the minimum of the cycle n+2. The connection between cycle duration and
its amplitude is established. Duration of the "latent" period of evolution of
extended cycle between reversals and a minimum of the current sunspot cycle is
entered. It is shown, that the latent period of cycles evolution is connected
with the next sunspot cycle amplitude and can be used for the prognosis of a
level and time of a sunspot maximum. The 24-th activity cycle prognosis is
done. Long-term behavior of extended cycle's lengths is considered.
