If the mob has a static spawn point
and fully resets, then it will forget you. (The static mob can still get social aggro back if other mobs have not reset or blurred. The static can also proximity aggro if you are too close)
If the mob is a roamer (or hasn't reset to a static spawn point), then you have to roll a FD mem blur chance which varies by level.
The length of time you FD will not mem blur the mob in and of it self.
http://wiki.project1999.com/Skill_Feign_Death