Actually, it depends.
Here’s how it works for non-federal-sector employees:
An employee must file a charge of discrimination with the U.S. Equal Employment Opportunity Commission within 180 calendar days from the day the discrimination took place.
However, if your business operates in a state or locality that has a fair employment practices agency that enforces a law that prohibits employment discrimination on the same basis, that 180-day deadline is extended to 300 days.
There are a few exceptions, of course.