It's been a long time since Real Madrid last allowed a coach to complete his contract, so Zidane confirming that he has a two-and-a-half-year contract with the club is most likely something meaningless. If he does well enough, he will sign an extension but if he doesn't, he will be out of the club before his contract is over, no question about that.
Zidane also said that "details of the contract are yet to be figured out," meaning that the parts have not talked about his salary now that he will be coaching the first team. It seems reasonable to think that Zidane's wages will be increased from the ones he had when he was with Castilla.
Zidane's Real Madrid will play their first game this next Saturday against Deportivo. The schedule is fairly manageable until Real begin the Champions League's Round of 16 against Roma, a tie that will be a test to see if Zidane has what it takes to manage the first team.