Lightning, I'm not sure how to answer that. There are so many variables, image resolution, render quality settings, coverage, etc...
I ran a couple of tests using a cropped strip from my "Worth the Climb" shared image. For this specific test, the TG grass only added about 5% to the render time. For the whole "Worth the Climb" image, the grass probably doesn't add more than 1% to the total render time but it certainly looks better than green shaded ground. I should also say that the grass populations have been restricted to the foreground area and do not extend to the grassy areas in the distance.