How big is your card? What format are you saving your images as? Your answer is that it varies by card size and type and size of images being saved. There will be a number indicated on the display that tells you how many images remain on the card, based on the current type of images being recorded.
Shooting RAW I get 529 shots on an 8Gb card... more than enough for most needs I would have thought. I wouldn't want to have more than about 500 on any given card in case of mishaps of one sort or another.