Getting from Amsterdam Centraal Station to Shiphol Airport?

I will be arriving in Amsterdam from Brussels (via train), and will have a few hrs to kill (about 5 hrs) before i fly to Krakow. So i need to know how long it takes to get from Amsterdam Centraal Station to Shiphol Airport, so i can know exactly how many hrs i will have to sight see.

It takes 14 to 17 minutes by train, 5 trains leaving per hour (peak-time).
You will experience it yourself, since your train from Brussels will stop at Shiphol Airport before reaching Amsterdam Centraal station.

LaFeeFan is correct. It's a fast and easy ride.

Note that you'll need to arrive at Schipol (correct spelling) two hours before your departure time.

If you arrive by train in Amsterdam, do not get off at the Schiphol station but stay on the train a little longer, about 15 minutes more at most.

You can leave your luggage in a locker in the station, and walk into town in a few minutes.

The trains to the Airport leave about every 10 minutes average, but not as nicely spread as that. But each of the lines serving the airport runs each 30 minutes, so that is the longest you can need to wait if things go normal.
The train will use about 15 minutes, a little longer for the one with several stops.

But do go to the airport early, check-in lines can be long, the line for passport control can take an hour and almost the same for the security check, and there is a longish walk to the gate as well as the boarding half an hour before the flight leaves. Best plan to be at the airport about 2 hours or more before the flight leaves.

You can find more about the trains on this site:
http://www.ns.nl/cs/Satellite/ns2007/en/mainmenu/include/1193053691834/travelers?p=1193053691834
It even gives the numbers of the track/platform the train will leave.

There is a direct train from central station into the airport. More tourist info on;

http://www.amsterdamtourist.nl/

Take about half an hour for the trip as the train might be delayed. You are required to be at least 2 hrs prior to the flight at the airport to get through customs, which may take an hour as it is high season and a very busy period for the airport to process all their flights.
Keep also in mind that some gates are a long walk from customs and they want you to be there at leat 45 to half an hour prior to take-off. I guess it doesn't leave you much time, but you don't want to risk the cjance of missing your connecting flight.
Depending on your citizenship, there might even be a chance you won't be able to leave the secure area of the airport between flights at all and cause you all the hassle dealing with customs even longer.

Good luck.