Canon is not disclosing the rated shutter life for the Canon EOS Rebel XSi / 450D. But in general, the Canon shutter for their budget models is about 50,000 actuations. This means that your shutter should roughly last 50,000 shots before it might start acting weird and need to be replaced at the Canon service center.
50,000 is just a rough guess in general. It could fail after 25,000 or keep going strong for over 100,000 shots.
How many actuations does canon 1000d before it will retire? I have a canon 1000d and a read elsewhere it will retire at certain actuations.