Back in the early days of Android, system updates were very random: they would roll out at different times, and often several times per year. Now, Google has taken a much more streamlined approach, releasing one major Android update per year and much smaller, security-focused updates once per month.
But if you’re not running a stock Android handset that is updated by Google—like a Nexus or Pixel device—who knows when, or even if your phone will ever even see any of those updates.