I left the screen on my DIY waterfall longer than normal, I left it 14 days ( 7 to 10 days normally ) between cleaning. Here is the result from both sides of the screen and a squeesed out ball of algae being weighed showing 120g of algae.
Now that is a good question. Although there is more growth it was not proportional as in there was not around twice as much growth more like 1/4 to maybe a 1/3 more.