Depends. Work-hour restrictions don't apply to fellows. They usually take call from home but may come in for complicated cases, procedures, etc.
Our vascular fellows work a lot of hours - they're always in house by 6:30 am, often have cases go late and always come back in for weekend cases, rounds and take back or emergent cases which go after hours. I vividly recall one fellow who did a 7 hour case on his birthday (which fell on a Saturday) - he was in his office doing some detail work on a presentation before going home to his family when I had to call him to report the ischemic foot on this same patient. He ended up back in the OR until nearly midnight (and still came in the next day for rounds).
Medicine fellows can often have long hours as well - especially the Cards, IR and GI guys. Since you don't have in-house call, you can have some days with shorter hours (ie, 12 hour days) and you may work less than some of your residents - again, depends on the specialty, the patient census and a bit of luck.