Research and analysis done by many psychologists shows that humans get an itch for significant change every 7 to 10 years. Some believe part of this is related to how our cells regenerate every 7 years. We change half of our main friendships every 7 years.
Obviously all this is a bit subjective, but based on the experience of most of us, this holds true. Something about our mind is “trained” to desire significant changes almost every decade, including our jobs.