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When leaving a company that has coddled you for years (think top offices, great food all the time, drinks, snacks, perks of traveling, people recognizing the brand, etc...), it can be a tough adjustment for the first two months. You might not miss the work, but you will miss the perks for sure.
Also, the two years salary requirement is a good rule of thumb. It gives you enough time to: 1) Relax, travel, and decompress for few months and 2) Start your own project, and see it to completion (any non trivial project will take at least one year).
I did something similar, and I think the author's advice is good.