When is benefits mandatory?

What is the name of your state? CA

I work at a place where there are no benefits given to the employees. No overtime, sick/vacation leave, health or dental even for those employees who have been with company for a year. How is this possible?

Overtime is not a benefit; payment of overtime for time worked is required by law at the Federal level and by every state. Overtime is required whenever it is worked. In California, with certain exceptions, overtime is mandatory and compiled on a daily basis. If you work more than 8 hours in a day or over 40 hours in a week, the employer MUST pay overtime. If this is not happening, you can file a complaint with the CA DOL. However, if what you are saying is that the employer does not provide the opportunity to work overtime, there is no requirement that he do so. The employer is free to limit overtime to zero if he so chooses.

As far as health insurance, vacation time, sick time etc., the answer to your question is, never. Nothing in Federal law or the law of any state requires any employer to offer these benefits to any employee regardless of how long they have worked. Your sole recourse is to find another job where these benefits are offered.

Edit to add minor correction; the state of Hawaii, and only the state of Hawaii, requires certain employers to provide health insurance. No state requires the employer to provide vacation time, sick time, or dental insurance.