Depending on who and how the deal is structured, they (industry/individual) seems to be hooked on the term of 90 days. Meaning 90 days from departure to rehire, for legalities to be wrapped up and contractual components to be satisfied from the past employer. If the incoming CEO is a "name" the complexity of his/her last contract and of his/her new one will be voluminous. Each data point will have to be vetted and checked by lawyers in out going and incoming situations. Either way, for some reason 90 days seems to be the standard and its equivalent to a business quarter so maybe thats the reason. Enough time to find a replacement and enough time to sign your selection.

I know of someone in the CEO position with a contract exceeding 150 pages.