I think it really depends on what you are going for?
I've only got a Large Diagphram Cardiod Condenser. For upfront lead vox i'll usually get them about 150mm out from the Mic. I have a pop sheild that sits at about 100mm so they are forced behind that.
but for breathy in your ear stuff I get them rigth up on the pop screen
if they are belting it out or I need more 'space' in the mix or for backing vocals they could be 300 - 500mm back from the mic or even further, II think there might be times when you'd want say almost 1m between the mic and the singer - for effect.
but I'm more of a hobbiest so I could be totally wrong.