I did it for a long time. My girlfriend and I went to university a fair distance apart (Cambridge and Edinburgh), and saw each other every...2 months or so, I guess, but that wasn't so bad - we were both working a lot (her more than me, to be fair) and having a good time with new friends in our respective cities. Spent more time together in the Summer, and that was fine the first two years.
The following year, I did my study abroad in China, and we kept it going through that too - it was her final year at Cambridge, so she was ridiculously busy, but we still kept in touch as much as possible, and she came out for a few weeks over Christmas and new year. It was incredibly hard, I admit, and I felt very isolated over there at times. Still, survived that, moved back to the UK, and she came to live with me in Edinburgh for 2 years.
Now, however, she's moved to Leeds to do a PhD, and I'm still in Edinburgh for a while before moving back to China, and we eventually decided we just couldn't do long distance any more, especially after living together. Neither of us was going to stop the other from doing what they wanted, and ultimately we knew this was coming, but I couldn't put my life on hold for 4 years and she couldn't come to China with me, so...that was it really.
It sucks, after that long, especially when we're still in the same country for now. Sucks even more to have happened out of circumstance, as I'm still so hung up over her - she was my best friend after that long too, and the change in dynamic is horrible.
Anyway, er - basically, it's possible, as long as you're both committed, but it can be very hard depending on how often you see each other and your own circumstances, and as I found, it can just be too much in the end.