How long does it take wine to ferment?

I took a 2 liter of Grape Juice and added a packet of Baker's yeast. It's been sitting for about 3-4 days. I've heard that the yeast will survive until ~15% alcohol content, which is about what I am trying to achieve. I'm not really concerned with taste or aging, just want some booze for when I'm bored. I had about 12 ounces last night and it felt like I drank maybe 1/2 bottle of beer so clearly the alcohol content is pretty low at this point, perhaps 5%. I added a cup of sugar to the mix in hopes of speeding up the process, if nothing else it will improve the taste at the end. How much longer will it take before the alcohol content is maxed. If it wasn't obvious why I'm going through all this trouble for what is almost surely going to be a shi*** tasting wine, it's because I'm not 21.