I made 4 gallons watermelon wine and I wanted to leave it a little sweet. So, when the brew was at 1.01 gravity I added 2 tsp of potassium sorbate. I stirred it well. Over the next few days there was still a lot of activity at the air lock. I let it go about a week. The temperature was 73f-75f. I checked the gravity again and it was, sadly, down to 0.990. I know I can add sugar to get the sweetness back but my questions is: Should I have added the sorbate earlier? How long does it take to work sorbate? Or is there some other trick to quickly stopping the fermentation?
Potassium Sorbate won't necessarily stop an active fermentation. What it does is prevent the yeast from multiplying. It is better used as a stabilizer once most of the yeast has dropped out and fermentation is essentially finished. At this point you can then back sweeten if needed.
Potassium metabisulfite or campden tablets would be a better option if you would like to slow or stop an active fermentation. When you add sulfites they make the environment undesirable for the yeast and activity will slow and eventually stop.
Another other option would be to cold crash the must by lowering the temperature to almost freezing and letting the yeast drop out and then adding your potassium sorbate.