I would imagine once it gets so bright, you go blind, that the threshold has been reached. (just kidding)
This is hard to pinpoint because as light gets brighter, your eyes (pupils) adjust accordingly.
There is a formula for this found in: IESNA Lighting Handbook, 9th Edition, New York, 2000.
Perceived Light (%) = 100 x SQRT(Measured Light (%)/100)
But I've read on this forum where people treat photonic brightness doubling in terms of electrical voltage and the 6dB rule, implying you must double the (measured) brightness of light twice before you see a perceived doubling in brightness.
Frankly, there are too many subjective variables to really nail this down.