When there is a chance they'll believe it.
If you are playing someone who calls everything, they are less likely to believe it, and there has to be something on the table the believe you've connected. Even then, if there is 999 in the flop, and you bluff, where the only reason someone would call hat is four of a kind 9s or a good full house, there will be people calling your bluff with a high card queen. Don't bluff against those types much at all.
That said, it's easy to say that, but not always easy to follow your own advice.