Last month my ticket was checked on the way back from Delft to Amsterdam, and that is only about an hour's trip. So, you never know.
The conductor spoke English, was very friendly, and asked me if this was my first trip to the Netherlands.
Here in San Francisco, our local Municipal Transportation Agency (SFMTA) loses lots of money every year from people who don't pay -- mostly young black kids who get on the buses and trains at the back door (illegal) or every brazenly just walk past the driver without showing a valid ticket. They seem to think they have a right to ride for free while others have to pay. Most of the time, the drivers don't even challenge them -- they don't want to get into an argument -- especially if it's a bus route that goes through the projects where there are a lot of gang members.
The other day, a kid got on my bus and flashed a transfer to the driver, then walked to the back of the bus. The driver yelled at him several times that the transfer was expired. No reaction. Finally, the kid held up a monthly pass for the driver to see, but I don't think it was his pass. He had same friends with him in the back of the bus, and I'm pretty sure one of them slipped him his pass for the driver to see.
There is a $75 fine if you get caught, but few get caught, although I've noticed that SFMTA is increasing its number of fare inspectors, who randomly board the buses and check everyone's ticket.
Compare this to the honor system in Amsterdam and other cities. There is a general culture of honesty there that we just don't have in the U.S. A shame.