Election Commission of India's Model Code of Conduct is a set of guidelines issued by the Election Commission of India for conduct of political parties and candidates during elections mainly with respect to speeches, polling day, polling booths, election manifestos, processions and general conduct. These set of norms has been evolved with the consensus of political parties who have consented to abide by the principles embodied in the said code in its letter and spirit. The Model Code of Conduct comes into force immediately on announcement of the election schedule by the commission for the need of ensuring free and fair elections. Much of it is designed to avert communal clashes and corrupt practices. For example, politicians should not make hate speeches, putting one community against another or make promises about new projects that may sway a voter.

When does model code of conduct come into force? Okie... think carefully first, then make a decision:
The relevant sentence in the passage is: These set of norms has been evolved with the consensus of political parties who have consented to abide by the principles embodied in the said code in its letter and spirit. The Model Code of Conduct comes into force immediately on announcement of the election schedule by the commission for the need of ensuring free and fair elections. So, the answer is immediately on announcement of the election schedule by the commission.