With a TY, anytime, the more the better, but especially for shoutouts; when someone's mentioned you or RT'd (Retweeted) you even.
And sometimes its good to give your faithful followers an #FF, a Follow Friday. The day when your outstanding twitterer pals, who have wowed you with their links, the wonders of the world, or plain down-to-earth common sense - an #FF is a great way to say thank you especially if they've re-tweeted everything you've been up to to their world . That's a bit like saying 'I love you' in public when your drunk - you know the 'I love you so mush, I do... (hiccup) come ere an give us a kish' mwa, mwa. You can't help but be overly fond of such folk.
Then there's the time you get a direct message - often there are folk out there sending automated messages saying TY for following me - probably because they get thousands, yes thousands of messages every minute and also because they've set up an account so that when they follow you 'it' messages you. Personally I never answer to an 'it'.
Twitter is about conversations, sharing knowledge , making connections. You want to make real connections, live in the real world right. Well I do, but sometimes you can take things too far...
If you're talking about where you want to go to dinner tonight with your nearest and dearest, it's probably best to Direct Message them.
And then there's how you use Twitter, a lot of folk out there are folk who will interact like fiends, they'll answer ever Tweet they get, and follow up and hound innocent bystanders. Whereas other folk will just tell you what they had for breakfast and never ever answer your direct message once, or follow you.
The point is what do you want to get from Twitter and what do you want to give? What are you sharing with the world? 'Be-ing the change you wish to see', is as good advice as you can get (thanks Gandhi); so if you think someone is twittering balderdash at you - tell them and be on your way, and if they're helpful, share that too. Every one wants to be validated right?
Follow me on Twitter if any of this has been remotely useful, TY folks