See this link to read the interview where Michael Schumacher answers questions about his return to Formula 1 competing for the newly-formed Mercedes GP team.
From the interview:
Q: Michael, welcome back to Formula One. Your signature on the contract has hardly dried - can you say something about the content of your contract?
Michael Schumacher: Yep, the deal was done only hours ago. Regarding the driving, the time frame is three years. There was never any intention to do a one-off. There will be an option after that to be brand ambassador for Mercedes, which would make sense in a way.