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.

You may be thinking of sweet ice wine. But the grapes used for making that are not only frozen at some point, but juiced while frozen. So the juice is a more concentrated solution of, well, everything that was in the grape before, because ice (from the water inside the grape) likes to form without any foreign molecules in the crystal lattice.

After thawing, though, everything is dissolved again in the same amount of water as before, so the taste should not change just because of that.

Edit: There are fruits that do change permanently in taste after freezing, even if thawed again (e.g. in sloe, the tartness is reduced). But at least to my knowledge, grapes do not belong to that group.

When you freeze grapes you drive water out of the fruit, all that is going to do is intensify the flavor that is already there. If it's sour to begin with it will be more sour, if it was sweet to begin with they will be sweeter.

As Matthias mentioned: you might be thinking about Sweet Ice Winem which, traditionally, is left on the plant til the first frosts hit, and freeze the grape. Micro-ruptures then allow some water to escape, concentrating the 'stuff' that is inside.

But these very late harvest grapes are high in sugar, because they have been left to ripen as long as possible. So it is not the freezing as such that makes them sweeter - the freezing just concentrates the sugar that is already there. If the grape was sour, the freezing should simply concentrate the sourness eve further.