Catnip is a perennial herb of the mint family. It is believed that the chemical nepetalactone is what triggers the response in the cat's brain. Not all cats respond to catnip and for the cats that do, the response is usually seen for only a few minutes at a time and then the cat acclimates to the scent. If the catnip source is taken away for a few hours or a day or two, then when re-introduced the cat will experience the same psycho-sexual "high".
As a rule of thumb kittens that are younger than 3 months of age don't respond to catnip, this may be due to their olfactory senses not being fully developed. And some cats, no matter what age, are just immune to the chemical. Some cats will eat the catnip and if a large quantity is consumed the cat may vomit or get diarrhea and therefore the amount of catnip offered should be limited or not offered at all.