When did the Latin Catholic Church begin to generally require fasting only one hour prior to the Eucharist?

To understand the full reason of when and why the Catholic Church changed the Eucharistic fast we must look into the whole history of receiving Holy Communion.

Pope St. Pius X (1903 to 1914) in his Sacra Tridentina of December 20, 1905 changed old traditional age of being allowed to receive Holy Communion and encouraged frequent Communion and not just only once a year as was the norm in his day.

At a time when most Catholics received Holy Communion only once a year as their “Easter duty,” the saintly Pope Pius X encouraged Catholics to receive Holy Communion more frequently and changed the age for First Holy Communion from the teenage years to the “age of reason” – 7 years old. - Liturgical reforms began before Second Vatican Council convened

Pope St. Pius X did not change the Eucharistic Fast at the time of his decree on the reception of Holy Communion: Sacra Tridentina. That came with Pope Pius XII.

In the 1950s some parishes started having evening Masses. These evening Masses gradually led to the changes that all parishes currently have with vigil (Sunday) Masses being permitted on Saturday evenings. This was intended to help Catholics with legitimate commitments such as hospital workers to fulfill their “Sunday obligation.” This is one reason I do not attend Saturday evening Masses unless I have an obligation of some sort which takes place at the same time of my Sunday mass.

Now taking into consideration that Pope Pius X allowed the reception of Holy Communion more frequently and lowered the age of receiving the Eucharist and that Pope Pius XII permitted evening Masses, the next step would be to shorten the Eucharistic Fast.

In 1953 Pope Pius XII changed the Eucharistic fast from midnight to 3 hours and in 1957 from 3 hours to 1 hour.

He was the first to permit Masses to be celebrated during the evening hours and also made significant changes in the Eucharistic fast (which for centuries prior to 1953 meant that Catholics were to refrain from food and drink – except water – from midnight until receiving Holy Communion at morning Mass). In 1953 the fast was changed to 3 hours from solid foods and 1 hour from liquids. In 1957 the conditions of the fast were more relaxed in an effort to support changing working conditions of the people and to accommodate the faithful desiring to attend afternoon or evening Masses.

As evening Masses became the rule rather than the exception, the 1 hour fast prior to receiving Holy Communion was officially promulgated by Pope Paul VI as a universal accommodation. This took place at the end of the third council session in 1964. Almost 10 years later a Vatican Instruction published on January 29, 1973 by the Congregation for the Discipline of the Sacraments called Immensae caritatis affirmed the 1 hour fast and gave additional accommodation to the sick. This eventually took the form of church law (canon 919) in the new 1983 Code of Canon Law. - Liturgical reforms began before Second Vatican Council convened

Wikipedia put it this way about the Liturgical reforms of Pope Pius XII:

Eucharistic fast

To allow for the faithful to receive Communion at Masses in the evening and at other times, Pius XII decreed several changes to regulations regarding the Eucharistic fast. Prior to 1953, Catholics were required to abstain from consuming any food from midnight before receiving Holy Communion. This requirement to fast from midnight was changed by the Apostolic constitution Christus Dominus, issued in 1953, by which, the fasting period was reduced to three hours from solid foods and one hour from liquids before communion. Water or medicines did not break the fast (AAS 1953, 15-24). Four years later, in the Motu Proprio Sacram Communionem (March 19, 1957, AAS 1957), the fasting requirements were further reduced.