I've taken every sleep aid ever made, and that is not a very good one if you are experienced with continuous dosages of such things. If you are new to sleep aids, then that is a very good one to start with, as it is not so potent as to throw you for a loop, but it will put you to sleep if you are indeed a newbie. As for the dosage, you should follow the directions on the bottle, and if you don't have the bottle, just ask your local pharmacist. How long it takes to kick in is entirely up to your BMI (Body Mass Index), or rather how much you weigh is a good measure. I am 175 lbs., and 5'11" tall, and it takes all pills approximately 25-45 minutes to kick in good. If the Tylenol P.M. doesn't do it for you, then I would recommend checking on muscle relaxes! They are what I use as a sleep aid these days, and that's after trying everything else. Not only do they make you very tired, they also relax your body completely. Stressed out all the time, and need to give your body a break? Then try those. I find a generic muscle relaxed by the name "Zanaflex" works extremely well, and it's relatively cheap. "Flexeril" is a great one too.
Tylenol usually doesn't make someone sleepy. It just relieves some pain and reduces fever. However if you take Tylenol PM it will make your drowsy. How long it takes just depends on your metabolism. It usually takes about 2 hrs for me but I know others who fall asleep within less than an hour.