Underwriter here who does this for a living and this is absolutely false. You do not need any specific length of time to use the full income if the employment is full time (40/hrs a week).
A borrower could have started his job yesterday, hell he doesn’t even need to have started just an offer letter stating he will start a full time job, is sufficient to qualify them at their current salary.
There is a big misunderstanding about the 2 year history. The 2 year history comes into play only if the income is variable, where the borrower does not work full 40 hours a week or overtime/bonus is needed to qualify and the income is averaged over two years. Even then there are programs that allow for 1 year averaging.