The Mercury Mahadasha is the period of Budha in the Vimshottari dasha, lasting seventeen years, and it closes the fixed sequence before the cycle begins again at Ketu. It follows Saturn. As the significator of intellect, speech, commerce, and skill, Mercury in its dasha is traditionally read as a season favouring learning, communication, trade, and dexterity, its benefic or mixed results shaped by the company Mercury keeps and its placement in the birth chart, for Mercury takes on the nature of the planets it joins. The dasha a person is born into is set by the nakshatra of the natal Moon. These are the interpretive conventions of the tradition; the Vimshottari system is a mathematical timing scheme keyed to the Moon's birth nakshatra, its nine periods together spanning one hundred and twenty years.
Mercury's seventeen years closes out the fixed sequence before it returns to Ketu and begins again, and because Mercury is classically held to take on the nature of whatever planet it is conjunct or aspected by, more than any of the other eight grahas, its mahadasha is traditionally read as the least fixed in character of the nine, its outcome depending heavily on natal placement rather than on any settled Mercurial theme in the way Sun or Saturn periods carry a more constant signature. The full hundred-and-twenty-year Vimshottari cycle repeating from Ketu again after Mercury means a single human lifetime, on the system's own hundred-and-twenty-year premise, is expected to complete at most one full pass through all nine mahadashas, with the specific starting point set, as throughout this system, by the birth Moon's nakshatra.