One to three nights is what people usually stay.

You will need a full day (and we mean full) just to take a good look at the Argentinean side of the falls.

You will need half a day to visit the Brazilian side.

 If you stay only one night try to book the correct flights in order to have more time at the falls.  If you have the chance, stay two nights and you will like it better. Seeing all in one day is possible, but can be pretty tiring and you will miss parts of the park.

We do not recommend trying to visit Iguazu Falls in only one day, as a day trip from Buenos Aires. You simply will not have the time to really see the place.