The general rule is a fist's-width away.
Make a fist, and put it up to your mouth like you're going to blow into it. the mic should be on the pinkie side of your hand, at least that far away.
the truth is there are no rules about how far you should be from your condenser..... if you vocals sound better when you're closer, maybe because of the proximity effect, then so be it...... if it sounds better backed of, then use that.....
There are no set rules... maybe start with the pop filter a couple of inches away from the mic and your mouth a few inches away from the pop filter... but the main thing is to experiment until it sounds best.