I delete once I've imported them and once I've had a chance to look at them on the screen, but it's either right away when I download the card or the next day if I'm too tired. Honestly I should delete more
. But I go through in Lightroom and delete the obviously over/underexposed shots and the ones that are massively out of focus. Then I go through and select the actual keepers. It's the ones in between the definitely delete shots and the keepers that I need to be a little more liberal with the delete key with.