Just to be clear: I myself am not a full preterist. I believe that the Second Coming of Christ is still future. He could return today, a year from now, a century from now, a millennium from now, a million years from now, a billion years from now, etc. We have no idea when.
I do not think that any particular length of time would ipso facto be too long for the “last days”. I think the term simply means “the last era before the Second Coming”. Before Pentecost in Acts 2, there were still specific things that had to happen before the Second Coming could happen. But once the Holy Spirit descended on the 120 believers (in fulfillment of Christ’s prophecy), it has been clear skies ahead. Nothing remains that has to happen before the Second Coming. We can no longer say, “Ah, the Second Coming can’t be this afternoon, because X hasn’t occurred yet.”
Now that we are in the era of “the Second Coming can happen at any moment because all the other prophecies have been fulfilled”, we are therefore in the last era, we are in the last days. The last days have lasted 1,986 years so far, and we have no idea how long it will be until they are complete.