it depends on what your goals are and how much motivation you have. your training program should reflect your training goals. a professional or aspiring athlete should train differently than a bodybuilder. someone interested in doing ironman/triathalon/decathalon type of events should train differently from someone interested in running marathons. etc. etc.
it depends on how hard you working out. if you work out very hard like strongman style, 2 days can be enough.