Going by 9292.nl/en (the Dutch public transport guide website, also available in English) minimum travel time from Rotterdam Central Station to Amsterdam Central Station is at minimum something like 40 minutes, and more like a full hour at minimum. Depending on the exact routes of course you would add quite some time due to needing to transfer to different busses, trams and trains. Dutch public transport is generally pretty reliable and cheap, if you don't have a car available and are going to travel a fair bit you should consider an anonymous or disposable public transport card for ease of use. There's a link here describing the particulars of that.
Going by car you should expect to take at least as long. And if there's traffic, it's going to take a lot longer.
Also, Dutch fuel is expensive, you can end up paying the equivalent of 2 dollars per liter of gasoline, or about 9 dollars per gallon. Seriously, consider using public transport, it's a bit of a hastle but it's really cheap if you aren't staying long and are only going to be in the major cities.
Going by car you should expect to take at least as long. And if there's traffic, it's going to take a lot longer.
Also, Dutch fuel is expensive, you can end up paying the equivalent of 2 dollars per liter of gasoline, or about 9 dollars per gallon. Seriously, consider using public transport, it's a bit of a hastle but it's really cheap if you aren't staying long and are only going to be in the major cities.