Most of the time, you hyphenate a compound adjective when it precedes the noun it modifies, like so:
I need a full-time assistant.
You do not hyphenate a compound adjective when it follows the noun it modifies, like this:
I need an assistant full time.
So, my friends, this is incorrect:
You don’t have to hire an executive assistant full-time to see results quickly.
It even sounds different: a full-time assistant; an assistant full time. You can hear the difference.
However, I would write it like this, for clarity:
You don’t have to hire a full-time executive assistant to see results quickly.
In fact, I think you can say this: Even New York Times best-selling authors could benefit from hiring top-quality editorial talent.
Just sayin’.