I have changed jobs often over 49 years, usually because the learning opportunities were not there. I was at Bell Labs/Bellcore/Telcordia for some 14+ years because the learning opportunities were strong. Leaving out the Bell experience, my average was 3 years: a few of 2 or less and a few others of 4-7 years. But there has also been the period of 6.5 years, in between working for others, when I have been (and am) and independent contractor. So 9 companies besides working as an independent. Oh, and 1.5 years as a musician.