When are the Jews in Judea supposed to flee to the mountains?
They are to flee to the mountains at the point in time when the Antichrist comes in, defiles the temple, and causes the Jews to stop their worship of their Old Testament system. This specific event is the signal for those in Judea to flee.
Les explains that 'until' and 'then' are time words, and now He says at that point in time when the Antichrist comes in and defiles the temple, and causes the Jews to stop their worship of their Old Testament system, then let them who be in Judea flee to the mountains. Les tells listeners to keep that in mind and refer back to it later. He then connects this to the remnant Jew in the Tribulation, noting that this portion of Scripture is not the 144,000 young Jewish men but an admixture of a whole society, including those on the housetop who should not come down to take anything out of the house.