From textbook type examples of MTBF [Mean Time Between Failures] in reliability. If two light bulbs are lit at the same time and the first one fails after one unit of time, then the second one will be estimated to fail at 1.5 units of time.
Only if you know or are assuming some additional information about the probability of light bulb failure.
FWiW, my first DSLR went through TWO shutters. Like the newer desgins or hate them, it was surprisingly straightforward for Canon to swap the old worn one out and drop a new one in. Turnaround was less than $200 and less than two weeks including phone calls, faxes, and shipping. I expect the shutters are near-identical with those in the late-model film EOSs... though it's hard to see up close without an opening film back. But my Elan looks like it's built around a shutter "unit."