For certified planes, it is required that the stall warning (if equipped) be sounding no more than 10 (MPH/Knots) before the stall, in that configuration, and must be sounding by 5 (MPH/Knots) before. It was my job, decades back to set this up on Cessnas upon which STOL kits had been installed, as the wing cuff installation requires entirely repositioning the stall warning sensor. When I was test flying the Caravan last December, doing stall testing in every configuration (for a modification approval) I found that the stall warning speed was too fast, indeed, sometimes I was getting a warning 19 knots before the stall break. The modification would not have been a factor in this, it was just the plane. I snagged it, and deferred it, as the Transport Canada test pilot was known to be coming to check a few of my test points, and I did not want to have adjusted the stall warning, and have to refly three hours of stalls to get new warning speeds for the report. I briefed the test pilot about this. He flew some stalls while I rode right seat, and got all the same results I had. so when the testing was done, I formally snagged it for adjustment. I offered to fly the adjusting testing the following day, but the weather was poor for the next few days, so I came home, and the company pilot did it. Every maintenance check flight I fly will include stalls to the break, both flaps up and down, to confirm handling, and stall warning operation.