The time in which the teaching/religion/movement remains in an unaltered state. Almost invariably, however, the way a new revelation from heaven works is that God will reveal himself in a generation, and then when the prophet/prophets of that time — the mortals living, the messengers — die, what survives cannot be kept intact. It simply cannot be kept in an unaltered, fully-preserved condition. You need another Peter or another Paul or another Moses; you need another one with that standing, or it falls into immediate disrepair. While there are living oracles that are in communication with God, that is the best definition of a generation. Uninspired men cannot add to the work of a prophet.1
1 500th Year Reformation Talk Two, 19–20, Dallas, TX, Oct. 19, 2017, Q&A, transcript.