Normally, the FG will be between 0.990 and 0.996 when fermentation is complete.
I don't know what a Tetra pack is, other than a 1 liter juice package. Is Sorbate on the ingredient list? That is a preservative, and if present may make fermentation difficult. Sorbate is what we use to stabilize a backsweetened wine so it doesn't start fermenting again. Most commercial drinking juices contain sorbate and are not good candidates for fermentation. However, thousands of people (including me) have proven that even with most unlikely commercial juices WILL ferment, even if not all that well.
Since there are added sugars, it may be that some are not fermentable, so the wine may be done.
Just saw
@Rice_Guy's response, and I fully agree. If it were me, I'd let it go another 10 days. If the SG doesn't change, it's probably done. Since it's under airlock, it's safe for a while, at least that long.
For bottling sweet wines, stabilize with Sorbate and K-meta. For 5 US gallon / 19 liter batches, the dosage is 2-1/2 tsp Sorbate (1/2 tsp per gallon for the brand I have) and 1/4 tsp K-meta.
Always read the label for dosage, as some brands may be formulated differently. Do not overdo the Sorbate, or you'll get unwanted flavors.
However -- if juice contains Sorbate, I have no firm idea regarding how much Sorbate to add. Off-hand, I'd add half the normal dosage and keep an eye on the wine.
Does anyone else have better advice?
You're making small batches, which makes dosing tough. If you can, I suggest starting with 5 liters of juice so you can fill a 4 liter jug. Put the remainder in smaller bottles with a small headspace. For 4 liter batches you can use Campden tablet, 1 per 4 liters, and the appropriate amount of sorbate.
Also, do your primary fermentation in an open container with a towel over it to keep out critters. Early in the process, yeast need oxygen for reproduction, and a sealed container inhibits that. Once a wine gets down to 1.020, it can go under airlock.
I have 2, 7, and 8 US gallon food grade buckets, as well as 32 gallon Rubbermaid Brutes for primary fermentation. Whatever you use, make sure it's food grade plastic, or if another container, it's safe to contain an acidic solution (wine is very acidic).