Man of Sin

Paul said the man of sin would sit in the Temple. 

The Temple still existed at the time Paul wrote, but not too many years later it ceased to exist. 

If Paul literally meant that Temple, like it seems, then the prophecy must have been fulfilled while that Temple still stood.

The prophecy didn't originate with Paul himself: he sourced it from the words of Jesus. 

Jesus had explained that the abomination causing desolation would be seen standing in the holy place, that is, in the holy place of the Temple. 

If Jesus literally meant the same Temple which still existed in His and Paul's day but which doesn't exist anymore, then it would seem Jesus' prophecy must have been fulfilled while that Temple still stood.

Paul's prophecy was linked with Jesus' prophecy. And Jesus linked His prophecy with Daniel's. 

Daniel foretold that an abomination would result in the desolation of the Temple and city and the scattering of the Jews. Jesus quoted that.

Since Daniel's, Jesus' and Paul's prophecies were linked with each others', it's likely therefore that they each had the same Temple in mind.

And that most likely was the very Temple which still existed in the first-century AD, which was then destroyed in that same generation.

If so that would mean Paul's prophecy has now already been fulfilled.

Even if Jews rebuild a replica Temple in future, it could hardly be the Temple which Daniel, Jesus and Paul had in mind, for the following reasons:

Because the sacrifices already ceased as it was prophesied that they would, with the destruction of the last Temple; and

Because according to prophecy, the scattering of the Jews was to follow not precede the abomination in the holy place - and that scattering already began with the destruction of the last Temple.

Another possible time-indicator:

Paul told the Thessalonians that they knew who it was who was still hindering that man of lawlessness from showing his colours. 

Did that mean the hinderer had been a contemporary figure of theirs? Otherwise how could the Thessalonians have known who he was?

Daniel and Jesus both seemed to say the Temple-event would happen within the generation of Messiah's first coming. Therefore that event had to happen before the second coming.

But as for the timing of His second coming, Jesus said no-one knows. Daniel, Jesus, Paul, the angels didn't know. Only the Father. 

No-one knows how much time would span between the Temple-event and the second coming. 

The timespan isn't of the essence, covenantally and prophetically speaking.

Messiah's first coming obviously had covenantal significance; and the Temple-event also had covenantal significance to Israel; and the second coming will obviously be significant. But the timespan between the Temple-event and the second coming doesn't in itself have any covenantal or prophetic significance. 

Could that be why they could speak of the second coming almost as though it follows directly after the Temple event. Because after it, nothing else of covenantal or prophetic significance needed to occur before Jesus can come as far as we are to know.

The whole Gospel-period is part of the same description.

There is indication of a timespan though no length is given.

But when He does come, it will be sudden. It will be quick. It will be unexpected.