I'm trying to make a 1.5 litre batch of sugar wine (kilju), how much sugar do I add to it?

Ultimately, the answer to that question is going to depend on what you want the final alcohol content to be. According to the wikipedia article on Kilju, the typical alcohol content is 5-7%. As you probably know, you can calculate the alcohol level using an alcohol hydrometer by measuring a before and after fermentation reading. Using this calculator, I got a 6.5% alcohol content with a specific gravity reading of 1.060 before and 1.010 after.

However, your question is related to how much sugar is needed to reach that specific gravity. This article indicates that 1 pound of cane sugar in (1 gallon) of water should yield a specific gravity of 1.046. This recipe for sugar wine looks pretty good, but it doesn't list the expected specific gravity levels.

In my mind, you are going to get the best results using an existing recipe and after you have successfully made it, start making changes to see what, if any, improvements you can make.