The Schwabe frequency band of the sunspot record since 1700 has an average period of 11.06 years and contains four major cycles, with periods of 9.97, 10.66, 11.01 and 11.83 years. Analysis of the O-C residuals of the timing of solar cycle minima reveals that the solar cycle length is modulated by a secular period of about 190 years and the Gleissberg period of about 86 years. Based on a simple harmonic model with these periods, we predict that the solar cycle length will in average be longer during the 21st century.
Read more here:
https://scienceofclimatechange.org/wp-content/uploads/Solheim-2013-PRP-Sunspot-Cycle-Length.pdf