If u take the intercity train from Amsterdam central station it should take you approx. 60 minutes. This train makes around 3 or 4 stops. If you take stop train you should add around 20 minutes more. Trains here in Holland are quite efficient and fast. Make sure you watch from time to time the train station board once the train reduce its speed Each station has the local name written above in big blue signs and along the walls. Rotterdam is the final stop so that should not be a problem. Welkom in Holland en veel plezier (tarboo behanaah).