A graha is not read in isolation but through the several tables that make up a Panchanga and a chart. It rules Friday in the seven-day week, which in the Indian tradition as in the European one is named for the seven visible grahas in the order of the planetary hours. It holds 20 years of the Vimshottari dasha cycle, and it is lord of three of the twenty-seven nakshatras, Bharani, Purva Phalguni and Purva Ashadha, spaced nine apart so that the nine grahas cover the twenty-seven exactly three times over.
The Vimshottari dasha system divides a notional lifetime of 120 years among the nine grahas in a fixed order, Ketu, Venus, Sun, Moon, Mars, Rahu, Jupiter, Saturn and Mercury, with fixed lengths that sum to exactly 120. Which period a person begins life in is determined by the nakshatra the Moon occupied at birth, and how far through that period, by how far through the nakshatra the Moon had travelled.
In the scheme of dignities it is exalted in Pisces and in its fall in Virgo, and dignity is what the classical rules for strength are built on, since a graha in its own sign or its exaltation is read as able to act and one in its fall as constrained.
Venus holds the longest of the nine dasha periods at twenty years, a sixth of the whole 120-year cycle. The lengths are not proportional to any astronomical quantity and are simply given by the tradition, which is one of several places where the Vimshottari scheme is a convention rather than a computation from the sky.