At the end of the week, I've already saved the company more than my salary. After a year, I am a critical asset to the company.
After 3 years, I've optimized everything, the ROI slows down, and I move on.
This pattern is not a tragedy. It doesn't crush your soul. It makes you a prized commodity in the business world, with a chance to make a change in your life every few years.
I have usually alternated between 3 years doing this, then 2 years doing startup work. It has given me a much broader base of experience and skill than anyone who stick to one one side of the coin. It also has given me years of experience in large industries (banking, energy, etc.) which I then use to find very real problems to which a startup can be applied.
I have led a very satisfying career, doing this for almost 20 years now.