That depends on the megapixel value of your camera and the settings you put on it. For example, my digital camera is only a 2megapixel that I bought over a year ago (NIKON Coolpix), and saves pictures ~700k, so I can take around 18-20 pics on a 16mb card. I have a 256mb compact flash card, so that allows me to take around 270 pictures before I have to download. I don't know what the memory is for your digital camera, but compact flash cards are cheap, even the faster data rate versions, you should be able to find a 512mb for under $90.
But, your camera should show you on the display how many pictures are remaining. My immediate guess, and suggestion, is that you spring for a larger memory card...