2. You perform these services while you are a nonresident alien temporarily present in the United States for a period or periods of not more than a total of 90 days during the tax year.
So two US flights per month and a long holiday there could have you over 90 days so the exemption wouldn't apply, and it looks like it doesn't apply anyway to aircrew.
A day means a calendar day during any part of which you are physically present in the United States.
So arriving at 23:50 on Monday night and leaving at 00:10 on Wednesday morning is 3 days.