14 Moreover, Spiritual Exercises, truly so-called, require a certain space of time for their fulfilment. And though, by reason of circumstances and persons, this may be reduced to a few days, or extended to a whole month; nevertheless it should not be curtailed too much if one wishes to obtain the benefits promised by the Exercises. For even as the salubrity of a place can only contribute to the health of the body of one who stays there for awhile, so the salutary art of sacred meditations cannot effectively benefit the spirit unless it spends some time in the Exercises.
Source: Mens Nostra (Vatican.va)