As for cardiology fellowship, at least where I train, the days or just as long if not longer than as an intern, (7a-5-7p most days), but you have more full weekends off. The work is a lot better though since you aren't just an IM "grunt" doing notes all the time. You'll be doing caths, echo, nuclear, consults, EP, CCU rotations as well as just a sprinkling of plane old hospital floor work. It was significantly more rewarding.
Probably more importantly, however, is what is like AFTER training. There is a saying in cardiology that "call is forever" meaning that almost all jobs will have call responsibilities. But unlike other fields, you're call will be busier given the nature of cardiology. And while you'll hear people talk taking a job without call and getting paid less, realistically there are few jobs without call simply because no one likes it, so they aren't going to bring someone on who isn't willing to take their share.
The days will definitely be 10-13 or longer (i.e. 7a to 5-7+) hours many days in practice since you will be doing much more than JUST covering clinic or rounding on patients. If you do something like interventional cardiology, then expect a much heavier call burden and later nights, usually somewhere from q3 to q7 depending on the group. I'm not trying to scare anyone, but let's be realistic.
All that being said, there are a few other things to consider regarding this.
1) most all specialist jobs in medicine require some form of call
2) the work is much more rewarding than just being a Hospitalist H&P/discharge summary jocky (hence why despite the good pay and ample time off, hospitalists have a 23% satisfaction rate)
3) cardiology is really the only field where you really cover the entire scope of practice (procedures, imaging, clinic, consults, CCU)
4) dealing with the heart is very rewarding since you are dealing with a very serious and complex organ system - no one gets super excited when their joint starts flairing again afterall
5) You will be well compensated for your work and dedication - remember the old saying "ya gotta pay to play"
Hopefully this helps put things into perspective