» When is the best time to bluff?

In the simplest possible terms, the best time to bluff is when you think your opponent will fold if you do bluff. Typically, you might make a bluff attempt when a face card falls on the turn after an all under card flop. You might bluff if a third flush card hits, or when your opponent shows weakness on later streets. Beyond that, it is nearly impossible to say when to bluff because bluffing is as situational and opponent-dependant as any play in poker.

The better question to ask might be, who should I bluff?? 

"You can't bluff an idiot," warns T.J. Cloutier. 

"Bluff the best player at the table," offers Mike Sexton. "He's the one who will lay down a hand."

Whenever and whomever you bluff, your betting pattern must make sense from pre-flop to the river.  Your play must tell a credible story.