Freud thought that analysis should not have a maximum or a minimum limit of duration whereas at the same time it is not permissible to be extended beyond the necessary time. Freud's patients were not analyzed for very long periods of time. Usually, their analyses lasted from several months to few years.
Nowadays, the analyses last on average much longer. Theoretically speaking there is no reason for an analysis to last for long other than to overcome the neurotic defences or other more difficult mental states which make their appearance during the analytic treatment.
In any event, according to Freud and the group of classical psychoanalysts, as long as the resolution of the Oedipus Complex, in all its complexity, has been achieved, the analysis should be considered as having reached a successful completion.