Wow, 16 hours? I suppose on a lower that might makes sense. I typically do 10 to 12 on a pistol frame. The biggest thing is not allowing it to heat or cool too quickly. I have a 1/2" thick pan in my air fryer that keeps the lower element from heating it from the bottom too quickly. I also start at 70c and go up 10c every 30 minutes until I'm at 100c. Those are just things I've tried that work and keep my parts from warping. Also good to moisture condition somewhat before assembly. Your part will be at its most brittle state right after annealing.