Rashi on Exodus 27:21/רש"י על שמות כ״ז:כ״א

1מערב עד בקר FROM EVENING TO MORNING — Give it its due measure of oil so that it may burn from evening to morning. Our Rabbis estimated half a log of oil as sufficient for the nights of Tebeth which are long, and they ordained a similar quantity for every night of the year, and if any were left over on the shorter nights it did not matter (Menachot 89a).