You just have to experiment. After setting the decay rate to anything else than "No Decay", you need to up the Intensity. You shouldn't judge the light's brightness in the viewports anyway. IPR is an handy tool for this.

On a side note:
- Linear decay means when the distance from light source doubles, the light has attenuated to half of original.
- Quadratic decay means when the distance from light source doubles, the light has attenuated to 1/4 of original.
- Cubic decay means when the distance from light source doubles, the light has attenuated to 1/8 of original.