How do you plan on sweetening it? If you plan on using a non fermentable sweetener then you can pretty much do it at any time. I would recommend secondary or after. If you plan to use plain sugar or something that is fermentable then it's best to wait till getting ready to store/mature the wine. Add at least 3tsp of potassium sorbate and 5 crushed campden tablets to a 5 gallon batch, give it a gentle stir and allow it to settle for 24/48 hours. After which draw off a measured sample (say 500ml), add 1/2tsp then have a taste, repeat until you are happy with the taste. Once you are happy simply multiply the amount of half tsp by 45 and you know how many tsp to add. Or to make it easier if you assume 1tsp is 5g just multiply 112.5 by the amount of half tsp you added and add that to the entire batch.
