Time Difference between 120 degrees phase shift signal or a 3 phase signsl
The time difference between the three (120 degree shifted) square wave signals depends on the frequency of the signals that you are trying to make.

The period of a single signal is:
\begin{equation}
T=\frac{1}{F}
\end{equation}

The time between the signals is 1/3 of this period because they are 120 degrees shifted, giving the following equation:
\begin{equation}
T_{offset}=\frac{1}{3}T
\end{equation}

In your case (50 Hz frequency) the answer is 6,67 millisecond. Your result is correct.