Have a look at this database, which should give you an idea. It looks like one can expect about 50,000 shutter actuations from an entry-level camera, and probably 4 or 5 times that for a pro model. http://www.olegkikin.com/shutterlife/nikon_d3000.htm
A way to avoid taking lots of extra shots is: when shooting on twos, set the playback speed to 12.5 fps and just take one shot instead of 2. Then duplicate in the computer.
I'm a "thrifty" guy. I try not to worry about this and figure I can cheaply replace a camera via ebay.