The Venus Mahadasha is the period of Shukra in the Vimshottari dasha, and at twenty years it is the longest of the nine, the largest single share of the hundred-and-twenty-year cycle. In the fixed sequence it follows Ketu and precedes the Sun. As the great benefic of pleasure, love, art, and comfort, Venus in its dasha is traditionally read as a long season favouring relationships, marriage, wealth, beauty, and the enjoyments of life, its actual results shaped by Venus's strength and placement in the birth chart. Which dasha a person begins life in is fixed by the nakshatra of the natal Moon. These are the interpretive conventions of the tradition; the Vimshottari system is a timing method keyed to the Moon's position at birth, assigning each graha a period of set length in a set order.
The mahadasha a person is born into is rarely a fresh, full-length period: what has already elapsed of the birth nakshatra is subtracted first. Each nakshatra spans thirteen degrees twenty minutes, and the fraction of that span the Moon has already crossed at birth is applied to the ruling planet's full dasha years to find the balance remaining. A native born with the Moon exactly at the midpoint of a nakshatra ruled by Venus, for instance, begins life already halfway through the twenty-year Venus Mahadasha, with ten years remaining before Venus's own antardasha sequence finishes and the Sun's period opens. This balance-of-dasha calculation, done from the Moon's precise longitude within its nakshatra at the moment of birth, is the first and most sensitive step in casting a Vimshottari timeline, since a small error in the birth time or in the ayanamsha used can shift which planet's period, and how much of it, a chart begins under.