Hmmm, it depends so much on what you mean by that. There are regular trains going to Amsterdam central station from the airport but then how long you need depends on what you have in mind. Station is across from the main part of the city centre (in practice, across from the red light district!) and within walking distance from the nicer and quieter canals towards Jordaan. There are also canal boat tours leaving from across the station. So I'd say it becomes worth going to town from the airport if you have at least 5 hours or so between your flights (meaning about 2.5-3 hours there if you want to be back 1 hour before the next flight) but you could easily make it a week and enjoy it a lot.