This was a few years ago (2004), but I had flown Continental MAN - EWR - CLE, on the cheapest ticket I could find (IIRC it was around 300GBP, fixed dates, cattle-class). I had intended staying six weeks, but in the event, stayed for the 90-days maximum allowed under the VWP.
Once I knew I wasn't going to be returning on the date on my ticket, I rang COA to see if I could change the date of my return and there was no problem - I had to pay $90 at CLE when I turned-up for the return flight, and that was it.