It depends. Just like anything else, there are different levels of quality available in suspension parts. The budget "value line" parts generally have an average life expectancy of about 30-50K miles. On the other end of the spectrum, there are actually upgrade parts available for many applications which can be expected to exceed the service life of the original (over 100K miles.) This is why it is so important not just look at price when considering a repair. Of course, the service life of any part is hugely impacted by driving conditions.