How concentrated is grape juice concentrate?

I have enough information to make a rough estimate. It would have been better had I measured one more factor.

I've now pressed the grapes, added the concentrate and some sugar (I'm aiming for about 13-14% so will add more sugar in stages). I ended up with about 4.5kg of grapes including skin and pips - I'm making red so they're still in there. Pressed, the juice only had 1.05 SG. Adding 2 bottles of concentrate, (1 red, 1 white) bright the SG up to almost 1.08, and I've now added my yeast. The concentrate was syrupy in consistency.

This means the concentrate brought in a lot of sugar. I'll only do a rough calculation here, with lots of rounding It's off the top of the SG charts I've found for juice (fructose), would have been off the bottom of my hydrometer, and I didn't think to weigh a known volume. It would appear that 500ml of concentrate delivered about 400g of sugar. Guessing at about 1100 SG as typical for a sweetish grape juice and going back to the tables, each 250ml bottle muse be equivalent to about 1 litre of juice, i.e. 4x concentrated. There's a lot of rounding in there, and quite a few big assumptions, one of which is that it really is just juice with water removed, but that I think can be justified from the label.

It will vary a lot. For instance one big difference between cheap, medium and expensive wine kits is how much liquid they give you i.e. how concentrated it is.

Is this is your product: https://www.wilko.com/wilko-red-grape-juice-concentrate-250ml/p/0483794 Reviews seem to claim it's for up to 20L of wine which seems a little implausible to me.

As mentioned in comments, a hydrometer is your friend. If you know what OG you are aiming for, you might be able to carefully add some of this to your must until you get the reading you want.