You would file 1040NR until they meet the substantial presence test. From IRS:
Link: https://www.irs.gov/individuals/international-taxpayers/substantial-presence-test
You will be considered a United States resident for tax purposes if you meet the substantial presence test for the calendar year. To meet this test, you must be physically present in the United States (U.S.) on at least:
- 31 days during the current year, and
- 183 days during the 3-year period that includes the current year and the 2 years immediately before that, counting:
- All the days you were present in the current year, and
- 1/3 of the days you were present in the first year before the current year, and
- 1/6 of the days you were present in the second year before the current year.
