I agree with Hoppy's points. Ultimately several experiments would be needed to figure out exactly the relationship. I'm sure it isn't linear.Someone would need to do the experimenting to be sure, but I'm 90% sure that duration and intensity are not even close to being equivalent. Plants all require a minimum intensity to grow at all, and no matter how long the lights are on if the intensity is below that minimum, the plants don't grow. Also, plants can only use light if they have all of the nutrients needed to grow at the rate appropriate to that light intensity. Adding more light, without those nutrients, all of them, does not cause the plants to grow faster.
But, it is possible that within a small range of intensities, more time can compensate for lower light.