A person receiving an advanced premium tax credit has a 90-day grace period to pay all premiums that are owed. If the amount owed for all outstanding premium payments is not paid in full by the end of the grace period, the insurer can terminate coverage. (45 C.F.R. § 156.270(d).)
Note, however, that under certain circumstances, your state may require insurers to extend grace periods to policyholders who temporarily cannot afford to pay their premium. This may apply during public emergencies or following natural disasters. Check with your state department of insurance to see if your state requires grace periods in certain circumstances.