The agencies responsible for enforcement of the Affordable Care Act issued final regulations on the ACA rule that requires employers to provide health insurance to eligible new hires within 90 days of the date an employee becomes eligible for coverage.  Group health plans and insurers may not impose a waiting period that exceeds 90 days after an employee otherwise attains eligibility for coverage. Eligibility conditions based solely on the passage of time cannot exceed 90 days.  Other conditions for eligibility are generally allowed, such as meeting licensing requirements, attaining a particular job classification, or successfully completing an orientation period (not to exceed one month), but may not be used to evade the waiting period limit. 

Additionally, an employer may require employees to complete a certain number of hours in order to become eligible for coverage, but the hourly requirement must be capped at 1200 hours.

When an employer cannot determine whether a new employee will be working full-time, it may implement an initial measurement period to determine whether the employee meets the plan's eligibility conditions.  However, coverage must made effective no later than 13 months from an eligible employee's start date (or, if the employee's start date is not the first day of a calendar month, the time remaining until the first day of the next calendar month).

However, California employers should note that California law prohibits waiting periods in excess of 60 days.  Like ACA's 90-day rule, this period is measured by calendar days and begins on the day that an employee first becomes otherwise eligible for coverage.  Coverage must start on either the first day of the month following the date of eligibility, or on the first day of the month following a 30-day waiting period.

The final regulations can be found at https://www.federalregister.gov/articles/2014/02/24/2014-03809/ninety-day-waiting-period-limitation-and-technical-amendments-to-certain-health-coverage.