the answer is, "it depends" there are way to many variables to know exactly how long you can dry hop before you start extracting flavors you don't want.
The same bet would be to not dry hop that long or it will be an experiment on whether it will bring vegatabal or grassy flavors in or not. I have dry hopped for 3 weeks too, but it was at a lower temp, like 55-60 and it turned out fine but I wouldn't do that at 70.