While you can't judge replacing shocks on a "rule of thumb" schedule, I think it is wise to at least put yourself in the mindset of the possibility of replacing shocks around 60k miles.
Everything that has been mentioned is correct....type of abuse on shocks, other suspension components operating properly, etc. However, shocks do have a lifespan; it's just not something that can be written on paper.
What I'm saying is that if it comes to a budgeting factor, I would plan to have the money to replace them around 60k miles. There is not a shock out there (other than a rebuildable unit) that is designed to last 100k miles. Whether you think it still rides well or not, the shock is not performing optimally at 100k miles.
Also keep in mind that shocks play a large role in the way your tires wear.