Your priest could answer this for you.
I seem to recall it was pretty sudden in our NJ parish. Then we stopped going.
There were a number of changes to the Tridentine that were done very gradually from 1954-1967. In 1968 in various places the changes became much more radical at a greater pace and by 1970 it was all over.
The Tridentine to Novus Ordo "evolution" was a gradual process which occurred over a 7 year period. Some would argue that it began earlier than 1962, and some will contend that the process is still evolving to this day. But to answer you question in a general sense, the process occurred between 1962 and 1969, with the Novus Ordo being officially put in use on the First Sunday of Advent, 1969.
The changes came in three obvious rounds, with a few modification in between too.
The first round was enacted in 1964 by Pope Paul VI's "Inter Oecumenici", and put into effect Jan 1, 1965. This was what is commonly referred to as the 1965 Missal. Many, including myself, will argue that this Missal was the true reform called for by Vatican II and no further reforms were needed per the council. This was still the Tridentine Mass, but without Ps. 42 in the beginning and the Last Gospel and Leonine Prayers at the end. Also allowed was a limited use of the vernacular, with the core of the Mass always being in Latin (e.g. Offertory, Canon).
An interim change occurred in 1966 allowing the Preface to be in the vernacular.
In 1967, the document "Tres abhinc annos" ushered in the second round. Here more vernacular was allowed, and many genuflections and signs of the cross were eliminated on the part of the priest. The Roman Canon was now allowed in the vernacular, thereby allowing an entirely vernacular Mass for the first time. But, this was still the Tridentine Mass, just in the vernacular and somewhat bastardized.
In 1968, the three additional Eucharistic prayers were published and allowed for use.
Finally, in 1969, the last round happened. All prior reforms were discarded in favor of a newly drawn up Novus Ordo.
You may want to read Michael Davies's "Liturgical Revolution". Book 3 of the series, entitled "Pope Paul's New Mass" gives a very detailed and accurate expose of the general changes I listed above.
I should add a few more points to answer your questions completely.
1. The use of vernacular was allowed but not mandated officially. Therefore, despite all these changes, the Mass could have continued to be all in Latin, with the only differences being the rubrics, calendar, etc. Hence, the true difference between the Novus Ordo and Tridentine does not rest in the language, but in the actually rites themselves.
2. Mass Facing the People - again something that was allowed (permission granted in 1965), but not mandated. So both the TLM and NO can be celebrated ad Orientem.
Same goes for Communion Rails, the choice of music, etc.
These are all accidental, but highly visible things which obscure the actual, official changes that happened.
Changes were being made to the Tridentine Rite prior to VatII, but they were minor and in fact many of the changes involved reviving forgotten practices in the rite. Things like the Easter Vigil had fallen into disuse in many places, for example. Also, there was a movement to revive Gregorian - which had also fallen into disuse and had frequently been replaced by very sappy hymns, such as Good Night Sweet Jesus (generally used at Benediction) and others. Choir singing, except in the German churches, was patchy to non-existent, and reviving choirs was a big part of the earlier reforms.
When VatII came along, most of us initially thought that it was just going to be a little more of the same. There was an initial "bridge" form (don't recall the years) that was basically a simplified Tridentine low mass, usually said in the vernacular, and then suddenly one day we walked into church and they were ripping the altar out, putting in a table, and giving us an unrecognizable Mass. I left the church in tears that day, and so did my husband. We just couldn't believe the travesty they had given us.
So the answer is that changes had been going on gradually, and they weren't always bad, and then there was a brief lull right after VatII, where the Mass had been dumbed down but was still not unrecognizable. And then suddenly the hammer dropped, somewhere around 1968 or so.
"The Mass of Vatican II"
http://traditionalromanmass.blogspot.com/
The author documents the differences between the 1962 and 1965, provides justification and citations for said changes, and critiques others. He is planning to address the issue of Tres Abhinc Annos as well, based on the author's words:
I think there are good reasons to reject the 1967 reform in total or in part. This will be covered in my critique.
The author of the study was born in 1984, by the way.
There is actually a video,that shows the whole story of what you are asking-it is called;What we Lost and the Road to Restoration- THE COST 26.50 -INCLUDES SHIPPING-
"In the Spirit of Chartes" Commitee
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The process of the change of the mass started in 1954 with the reforms to the missal during Holy Week, but still these were minor reforms, the 62 missal added St. Joesephs name tothe canon and took away the 2nd confetior. In the late 50s, mostly in Europe, but by the early 60s in some parts of the US, albiet only in a few locations, some altars were turned around, and in some of these places, the altar rails were taken away and communicants started to stand for communion. Mind you this was before Vatican II.
As other posters have stated, the change to having some of the mass in the vernacular, and also the addition of lay lectors(all male) took place with the 65 missal, and between 64-69, a large majority of parishes in the US turned the altar around, and probably a majority of the parishes by 69-70 were standing for communion.
So in terms of rubrics, by the time the 65 missal was in its last days in 69-70, the mass was not too far off from the Novus Ordo we see today. That said, even in the 70s, the Novus Ordo was still more conservative in many ways than we see today. There was no communion in the hand untill 77, and there were largely no EMHCs untill the late 70s, and it wasnt impossible to find a parish that still used the rail back then, also, untill the late 70s, and this was because of dissent, there were no altar girls and communion under both species was rare.
The way I view it, the decontruction of the mass was not just one event, but a 30 year time frame where the "reforms" took a major turn in 64-65 untill 94, when altar girls were allowed.