sseveninches:
What verse says or alludes to the desolation being only 50 years?
The period you're asking about is described at Zechariah 7:1-5. These verses refer to the fourth year of Darius (518BCE). (This is Darius I aka Darius the Great, not to be confused with Darius the Mede.)
In that passage, two men are sent to ask the priest if they should continue fasting to commemorate the fall of Jerusalem. The response they get 'from Jehovah' is, "When YOU fasted and there was a wailing in the fifth [month] and in the seventh [month], and this for seventy years, did YOU really fast to me, even me?".
These fasts marked the fall of Jerusalem in the fifth month, and the death of governer Gedaliah in the seventh month. The passage states that the question was asked in the ninth month, after they had already held the fasts for 518BCE. The response they received specifically states that they had by then observed 70 years of these annual fasts.
Because 518BCE was the seventieth year of fasting, this means that the first year of fasting , the year Jeruslaem fell, and the year for the beginning of the 50 years of desolation until the Jews' returned to rebuild the temple, was 587BCE. Note that although the Jews were released and arrived in their cities in 538BCE (Ezra 3:1), they gathered to rebuild the temple the following year in 537BCE (Ezra 3:8). (The JWs' claim that the Jews were released in 537BCE is a lie.)
The fifty years of desolation you're after runs from 587BCE when Jerusalem was destroyed, until 537BCE when the temple foundations were rebuilt.
Any questions?