They don't become effective at one defined airspeed. They are effective in proportion to the dynamic pressure, which is proportional to the airspeed squared. At a very small airspeed, they are effective but perhaps not effective enough to control the aircraft, depending on what you do. At twice that airspeed, they are four times as effective... which might just be enough.
It is like asking "at which temperature does cold water turn into warm water". There's no answer as it all depends on what you are trying to use it for!
I thought I was doing good trying to avoid those airport hotels... and look at me now.