PHS Act section 2708 provides that, for plan years beginning on or after January 1, 2014, a group health plan or health insurance issuer offering group health insurance coverage shall not apply any waiting period that exceeds 90 days.
Short answer: 90 days from the date of hire.
NOT first of the month following 90 days! This was allowed under the small group regs in Texas before the ACA rule went into effect – maximum 90 day waiting period with covered effective the first of the next billing cycle.
Carriers may require the effective date of coverage to be the 1st or 15th of the month (to align with the group’s billing period), so in practice the longest waiting period offered by most fully-insured small group carriers is 1st of the month following 60 days from the date of full-time employment.
The same rules apply to large group health plans.
Here is the final rule on the new-hire waiting period: https://www.federalregister.gov/documents/2014/02/24/2014-03809/ninety-day-waiting-period-limitation-and-technical-amendments-to-certain-health-coverage