At what speed is a spoiler effective on a car?
Spoilers and wings are different things, and from the question, I think you mean wing. A spoiler is a device used to “spoil” any turbulent airflow and reduce drag. This is what you find on NASCARS. A lot of passenger car spoilers are just there for the looks and don’t improve performance.
On the other hand, wings increase drag by providing downforce. They are used on the fastest road cars as well as racing cars like Formula 1, LMP1 and GT racing:
Spoilers will always reduce drag at any speed, but the effects at low speeds are minuscule because there isn’t much drag to begin with. The drag force is proportional to the square of speed, so they have larger effects at high speed.
Wings generate downforce (and increase drag) at any non-zero speed relative to the air, but again, the downforce is proportional to the square of speed. Below 60mph, wings have little effect apart from increasing drag and reducing fuel economy. It’s only at high speeds that they really come into play. However, some wings are designed to flatten their angle at very high speeds (over 200mph) to reduce drag for a higher top speed. This is prominently seen on cars like the McLaren P1 (above) and Bugatti Veyron.