@Tieup1: well, whenever I write a story (not only tie-up stories) I try to keep everything as realistic as possible (within the context of the story of course, so if there's magic available, there might be a magical solution to a problem) albeit sometimes improbable. So if the captive really has no way to break free without help, he or she will stay stuck.
@FelixSH: hmm, well, I guess it should be possible to untie the knots, but it might take a long time depending on the type of knots used. Well, since the stories are fictional anyway, I might play around with that a bit.
@drawscore: too bad I often prevent that situation in my stories by tying a rope around the elbows.
@Reidy: well, I want the characters to try to escape so they can be busted by the captor at the very last moment. Then they can be tied up even tighter and/or punished.

@ParanoiaDoesBondage, @Kyle: so if the captor is careless or in a hurry, the captive has a good chance to escape. That seems to make sense. ^^