How you write a millionth as a decimal is always the same, regardless of what unit of measurement it's in. The "1" is six places after the decimal point when writing out one millionth. This is easy to remember because one million in numbers is followed by six zeros. You can also count it out if it helps. The first place is the tens place, then hundredths, then thousandths and it goes up by a factor of ten each time. Ten, hundred, thousand, ten-thousand, hundred-thousand, million! Six places!
However, once the issue of units has been brought up it's usually possible to find a shorter way to write it using a smaller (or larger, in other cases) unit of measurement.
1 second = 1,000,000 microseconds
So a millionth of a second is a single microsecond. There are plenty of ways to write a millionth of a second, depending on which unit of measurement you want to use. All of the following are equally correct (though not equally practical), you can pick whichever you'd like.