I'm a property security guard. I work 12 hours every day and have four days off. Is this a violation of the labor law?

It must be illegal. You can refer to the following provisions to make a complaint.

Article 36 of the Labor Law The state practices a working-hour system in which laborers work no more than eight hours a day and no more than forty-four hours a week on average.

Article 90 If an employing unit violates the provisions of this Law and punishes the illegal extension of working hours, the labor administrative department shall give it a warning, order it to make corrections and may also impose a fine.

Article 91 Where an employing unit infringes upon the lawful rights and interests of laborers in any of the following circumstances, the labor administrative department shall order it to pay wages and economic compensation to the laborers, and may also order it to pay compensation:

(1) Deducting or delaying the wages of workers without reason;

(2) Refusing to pay overtime wages to laborers;

(3) Paying workers' wages below the local minimum wage standard;

(4) Failing to give economic compensation to the laborer in accordance with the provisions of this Law after the termination of the labor contract.