The time in devachan (spirit-land) is not of equal length for all human beings. The uneducated savage who has experienced a little of this world only who has applied his mind and sense only a little has a short stay in the devachan. The devachan is basically supposed to elaborate what the human being has learnt in the physical, to unfold it freely, to make it suitable to a new life. The human being, who is on a higher level of existence who has collected rich experiences, has to process a lot and, hence, has a long stay in the devachan. Only later, when he is able to look into these states, the stays become again shorter up to the point where the human being can immediately walk after death again to a new incarnation because he has already experienced what is to be experienced in devachan.
Source: Rudolf Steiner – GA 53 – Origin and Goal of the Human Being: Lecture VII: The Spirit-land – Berlin, 17th November 1904