Is the "last" generation counted from 1948 or 1967?
Since Daniel's "times" prophecies were not both fulfilled to cross-confirm each other until 1967, I would say it wasn't until then that we entered what Daniel's prophecy called the "time of the end". Same with John's 42 "months" problems cross-confirming his 1260 "days" problems. The "times of the Gentiles" in Jerusalem were not fulfilled until 1967, and that is one of the things in that passage that had to be fulfilled.
http://www.beholdthebeast.com/matthew_24_olivet_discourse.htm#times_gentilesRegarding ideas around the term "generation", a few different Greek terms are used, and there is a bit of the devil in those details!
http://www.christianeschatology.com/partial_preterism.htm#generationIn the verse you allude to, it is likely literal, but could even be metaphorical.
Strong's 1074
genea1) fathered, birth, nativity
2) that which has been begotten, men of the same stock, a family
a) the several ranks of natural descent, the successive members of a genealogy
b) metaph.
a group of men very like each other in endowments, pursuits, character1) esp. in a bad sense, a perverse nation
3) the whole multitude of men living at the same time
4) an age (i.e. the time ordinarily occupied be each successive generation), a space of 30 - 33 years
Could be referring to the generation of Christian men that all share the same "endowments, pursuits, character".
If literal generation is meant, which is more likely, it could be as long as a generation lasts today. Because of medical advances scientists speculate that people under 50 who take reasonable care of themselves may expect to live to be 120 (same limit as Genesis). Psalms suggests 70-80 year life spans.
Regarding the big touchstone of preterism that they like to stick futurists in the eye with:
http://www.christianeschatology.com/partial_preterism.htm#matthew_24_34Matthew 24:34
Verily I say unto you, This generation (genea)
shall not pass, till all these things be fulfilled.The literal generation sense of genea could be meant for this verse, since it is not a rebuke as the others explored at the prior link, but rather more informational, perhaps in answer to the earlier question regarding the end of the age that was to come after the "times of the Gentiles" in Jerusalem were to be fulfilled. While a million Jews were killed in 70AD, the Roman Gentiles killed another one half to three quarters of a million Jews less than 70 years later - with Gentiles ruling Jerusalem until 1967 - making it conspicuously untenable to suggest that the "times of the Gentiles" in Jerusalem were fulfilled in 70AD.
When we look to Strong's and a TR Greek/English interlinear we find that Matthew 24:34 can be as easily understood as indicating:
Matthew 24:34 Verily I say unto you, {the same} generation shall not pass, till all these things {shall have taken place}.
The Greek houtos {hoo'-tos}
translated as "this" is translated as "the same" on 28 other occasions. Since within passage Jesus included "Jerusalem shall be trodden down of the Gentiles, until the times of the Gentiles be fulfilled" as one of the things that needed to take place, and Jews did not gain control until 1967, that verse would indicate the passing of
a generation born no earlier than 1967.
Here's some speculating:
https://gracethrufaith.com/ask-a-bible-teacher/a-generation-or-a-lifespan/
