Talmud for Shevi'it 1:1
משנה עד אימתי חורשין בשדה האילן ערב שביעית בית שמאי אומרים כל זמן שהוא יפה לפרי ובית הלל אומרים עד עצרת וקרובים דברי אלו להיות כדברי אלו:
From when may one recite Shema in the evening? From the time the Kohanim go in to eat their Terumah (produce consecrated for priestly consumption). Until the end of the first watch, says Rabbi Eliezer. And the Sages say: Until [astronomical] midnight. Rabban Gamliel says: Until the break of dawn. It once happened that his [Rabban Gamliel’s] sons came from a house of feasting. They said to him: We have not recited Shema. He to them: If dawn has not broken, you are obligated to recite it. And it is not only in this case that they said it! Rather, in all cases where the Sages said "only until midnight," the obligation remains until the break of dawn. [e.g.] Burning the fats and limbs [of the sacrifices, on the Temple altar] — the obligation is until the break of dawn. [e.g.:] All [sacrifices] which may be eaten for one day — the obligation is until the break of dawn. If that is so, why did the Sages say, "until midnight?" To distance a person from transgression.
Jerusalem Talmud Kilayim
It is not quite clear what the minimum distance is; cf. Chapter 4, Note 2. The cubit of 52.6 cm would give 210.4 cm as minimal distance of two vines. R. A. H. Nach defines the hand-breadth as 8.5 cm, which implies that 4 cubits of five hand-breadths each are 170 cm. Modern viticulture works with distances of 135 to 150 cm; one is best off by adopting the minimal interpretation of cubits..