Depends on the time and place obviously, but whenever it's socially acceptable, mainly with friends, I swear almost constantly. It pisses me off when people take offence; after all, they are just words, and unless directed against someone then there's nothing wrong with it.
And the argument that swearing demonstrates a limited vocabulary is ridiculous, principally because there are NO words in the English language that are an appropriate substitute for a proper curse. Yes, you could say "oh, sugar" if you hurt yourself/somehting goes wrong. And consequently look like a twat.
EDIT: And I don't draw the line at any swear words, since to me they are all just words. That said, there are some I use with greater frequency than others, and it seems to vary; sometimes I might say bollocks quite a bit for a couple of months, then there are periods when I won't say it at all, and use others words instead.