The (larger) company's that I've worked with tend to be around 1 month per year of service. At an executive level I wouldn't feel the least bit shy about asking for twice that, particularly if you have a solid relationship with the CEO and can make a logical claim to the money.
Since this is resulting in either a major life change or a major amount of money, take Vlad's advice and have a quick consult with a lawyer.
Good Luck!
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