How much does freezing grapes longer sweeten them more?
The time doesn't matter. What you need is a single freeze-thaw cycle. Once your grapes have been frozen solid, holding them frozen won't do anything more. 

I get it that you are not happy with the results you got so far, but that is just because your freezing idea is not really suited to your situation. I have never tried eating thawed bananas, so don't know how large the effect is there, but the article states the two reasons for more sweetness: juices leaking out of cells, and amylase converting starch to sugars. First, grapes are a juicy fruit, unlike bananas, and you easily have access to their juice once you bite into them. Second, grapes have practically no starch. 

The problem with your grapes are not that they are a mealy banana, it's that they are sour. The acid is not going anywhere when you freeze them, and you can't create more sugar out of nothing. They will stay the way they are, freezing or no.