If that CEO happens to be a controlling shareholder who wants his job back, he would have to nominate a new slate of board members and schedule a shareholder vote according to the corporate bylaws (that are in turn regulated by the state of incorporation), which is a process that could take months to finish.

Presumably the new directors that the ex-CEO elects would hire him back, though it is worth noting that the directors have a duty to represent the best interests of all shareholders, not just those who voted them in. So if the CEO was fired for legitimate reasons and the directors act according to their duties, the ex-CEO should not be hired back.