That greatly depends on the individuals involved, how distant they are from each other and for how long they will be separated. If you are in similar time zones it is a lot easier to coordinate times in which to speak via Skype than couples that have 6-7 hour time differences. They only work when the couple truly love one another and see a future together. If one of them has any doubts then they should consider finding someone else.
I am married and currently dealing with a long distant relationship. If you have any more questions feel free to ask.