Texting weirdly enough works well for me, as does Whatsapp (anyone unfamiliar with it; it's an app for chatting through your cell, though it goes over the internet, instead of through texting). I should add, that it does depend a bit on whom I'm texting with.
I text with 2 people; my girlfriend and a friend. With my girlfriend, she, just as well as me is fine with just dropping in every now and then sending a message. If we really need to talk something over; we'll use MSN or just meet up. But she doesn't get all jumpy and annoyed why I don't respond and neither do it. With my friend; we just text about meeting up, I never have conversations with him through my cell.
So if I'm looking at "how am I supposed to tell when a conversation is over"... I have pretty much no training with that, lol. Both people have similar ways of keeping to conversation with me. In a way it helps that they don't have that need as well... chances are either of us would get annoyed if the other kept rambling.
If I meet up with me friend; we sometimes stumble upon situations where we're walking back towards his place (or mine) and we usually part ways there (since he lives around the corner), but we often stick around at either ones door for another 30 minutes, since we misjudged time in a way. Either of us has stuff he wants to ramble about and we're actually about to go our own ways again... we either walk another block or just see where it goes from there.
With my girlfriend when she's over... well, I'm not sending her home nor telling her to shut up. Though similarly, she's fine with me going to attend some of my own interests if she's around while she does her own stuff. She's still around, but that doesn't mean we have to keep talking.
Here's an interesting thought; Telling someone "bye" as sort of an agreement that you're both parting ways, should be a mutual agreement. All to often, this is not the case, since either will want to talk some more. So there actually is no agreement and either one is just "walking out".
Maybe I'm a bit rational here, but quite often I'll clock my conversations in terms of "you have 5 minutes". And similarly, if my girlfriend comes over I'll tell her upfront what time I'm walking her to her train. It's a way to avoid dull moments, and an agreement of sorts when "our time is over". That worked pretty well so far. One time, someone stopped me in the streets, trying to sell me something and it went on something like this;
She: Can I ask you something?
Me: Well, you already did... but ok... you have 1 minute, starting now...
She: *rambling about a product* then, a small silent break
Me: You done? You still have 15 seconds
She: Oh no, there's more I should tell you
Me: Well, your minute is over, sorry they don't train you to market your product faster, bye!
And I'm gone.
And I can be like this a lot. Even with my parents; Can I ask you something? Yeah, sure... I'm leaving in a minute, hurry up. And I will walk out, I don't care. They have been told up front.