i think it depends largely on personality and needs of both partners.
i need to be with the person i love on a physical level (i don't mean i'm a nympho, lol, but i need to be able to hug them, hold their hand, talk to them, and look into their eyes not via webcam, more than once in a blue moon), and i find it very very hard to maintain a relationship with a lot of distance for a long period of time (with my long distance thing, i quickly started loosing it on so many levels, and had to up and haul myself halfway round the planet, to be with him lol, if i hadn't of been able to do this, i know that realistically my relationship wouldn't have survived much longer).
i know of some people who are very busy, very independant, very strong, or for whatever reason just very capable of being with someone, while being far away from them for long periods- but in most relationships like this that i can think of, partners know they will be with their loved one for at least some good lengthed, and fairly well schedualed periods of time in the quite near future- eg: those with partners who work away from home in the forces, work on oil rigs, drive trucks long distance, etc, or they have some plan of being together one day, and are consiously working on it.
i couldn't see them being able to maintain this perminantly if they knew there was little to no hope of seeing/being with their partner at all... but i guess those who marry murderers on death row do it. must take a special kind of person.