Mercury mahadasha lasts for 17 years, and wherever Mercury is placed in your birth chart it will light up like a light bulb along with the houses he rules (Gemini & Virgo), so situation of those houses will come alive including the placement of Mercury. But, what really does take place in this mahadasha? Mercury is our intelligence, communication, studies, writing, speech and are ability to do business. In the time period of Mercury one will tend to suddenly focus on learning, going back to school, improving upon their skills, or getter a better degree to get a promotion at work or gaining profit in business deals. There is a lot of need for communication in Mercury time period. One suddenly finds the urge to speak, write and to be heard in Mercury time period. Since it's also communication, all aspect of communication can rise, from being a journalist, anchor, tv reporter to even a lawyer, because being a lawyer also means having the ability to communicate. There are a lot of dealing with siblings, close neighbors and friends.
Mercury dasha isn't really about gaining wealth, marriage or children, because any dasha can bring such events, it all depends on what house a planet controls and where exactly a planet sits. If Mercury is exalted there is a lot of gain through the house where Mercury is sitting. If Mercury is in the 2nd house then through speech, public speaking, taking on family business, opening up chain stores, or even banking can provide the souse of huge wealth since 2nd house represent wealth. The native would also know how to think logically in such dasha and make right analysis about the projects they taken on. But, if Mercury is debilitated, even small tasks would sound like a chore, even small tasks would take a long time to star because Mercury becomes a procrastinator in the sign of Pisces since he is forced to think through emotions than logical thinking which Mercury always uses. However, if Mercury is in kendra, especially 10th house, then creativity can be a souse of trade by being deblitated, business and intellectual pursuits, the native would shine through their sheer imagination as Albert Einstein did. He had Mercury, Venus, Sun and Saturn in the 10th house of Pisces, which enabled to him to think through his mathematical problems through imagination and creativity. His Mercury receives a neecha banga by being in kendra, sitting with exalted Venus while in the most powerful kendra house 10th house. During such mahadasha he came upon the theory of relativity. This is why it's important to see exactly where the planet is, because even a debilitated planet can provide such a house leap to a person.