All I know is when I first started I just did what I was told to do in my training. I would feel the carpet and I would describe it as slightly damp. After a few weeks and no one was ringing up 2 days later saying their carpet is wet and stinking I knew that if I had that amount of dampness I was OK.
So really it's experience over time that is your best guide.
P.S. I'm drying a thick chinese superwash now, it feels dry to the touch, but I know from experience that it will be at least another 6 hours with the blowers on it before it's ready to roll.