When did Christians switch from an ‘immediate baptism’ to ‘after years of instruction and scrutiny’ baptism, such as in the Apostolic Tradition? According to this article ‘Catechumen’ means one receives instruction from “a catechist in the principles of the Christian religion with a view to baptism”. I understand that some churches have this before baptizing people. I do not criticize this practice, but I am curious when this idea may have developed. In the scripture people are baptized immediately upon believing and there does not seem much delay in the matter. For example:


  But when they believed Philip as he proclaimed the good news of the kingdom of God and the name of Jesus Christ, they were baptized, both men and women. (NIV ACTS 8:12)


The Apostolic Tradition was the work of Hippolytus, written somewhere between 215 and 400 AD. Recent scholars seem to take the later date (source). The whole writing can be found here: Apostolic Tradition.

Among the 'oddities' of this Tradition, seems to be that people were baptized only after three years of catechism. 


  17 Catechumens will hear the word for three years...When the teacher finishes his instruction, the catechumens will pray by themselves, separate from the faithful...After the prayer, the teacher shall lay hands upon the catechumens, pray, and dismiss them. ...When they are chosen who are to receive baptism, let their lives be examined. (Hippolytus, The Apostolic Tradition)


I am wondering is there any evidence that this switch occurred before 215-400 AD, or not? Do some churches still practice a three year regiment?
According to A history of the catechumenate: the first six centuries by Michele Dujarier (Sadlier, 1979), the development of a longer initiation period predates Hippolytus (assumed to be writing in 215AD) by at least a few generations.


In Rome, the First Apology of Justin Martyr (155-157AD) attests to preliminary instruction in the faith, prior to baptism (though not necessarily for three years, and it was not organized systematically). Dujarier writes, "Some Christians dedicated themselves more particularly to the task of awakening faith and of teaching and opened 'schools' as did philosophers of this period. These were private initiatives and were not institutionalized. The hierarchical Church had not yet assumed the direct responsibility for the teaching given in them. But the fact is clear: the laity themselves carefully ensured the evangelization and instruction of converts. [...] The witness of Justin thus manifests the essential aspects of stages and, catechumenal requirements. In the course of the following decades, these stages and requirements were codified more and more strictly."
In Alexandria, there was a school of catechesis (using that word!). Jerome says it was founded by the apostle Mark; at least, it was in existence in 176AD. Clement of Alexandria taught there from 190AD. There is some doubt, however, about whether the term catechesis here should be understood as including pre-baptismal instruction, or whether it refers to teaching in general.
Other writings around 200AD refer to catechumens, proselytes, auditors, recruits, or novices. This includes work by Tertullian and Cyprian. Dujarier deduces the existence of a catechetical period in Carthage in 200-210AD from this history.


The Apostolic Tradition is therefore recording a more formal version of a method that had been happening for a long time - certainly since the time of Justin Martyr. The justification seems to be both pastoral and practical; it is not only that new converts should be truly converted in their lives, but also relates to the effort to ensure orthodox belief, in the face of various heretical movements of the time. After Constantine, a further reason was that some people were seeking to become Christian in order to gain social status (condemned by Augustine, The first catechetical instruction 5), so it was important to distinguish between real converts and pretend ones.

As far as current practice goes, I don't know of a church which specifically holds to a three-year period before baptism, though it is unusual not to have some period of instruction (at least in the case of adult baptism). There is nothing particularly special about three years, though; and as Dujarier records, that length was not universally observed, and the span of time is less important than the content of the instruction.