The average lifespan of a personal computer is a debatable topic simply because technology is advancing at a rate that does not match the demand on the market. New components are literally appearing on the market from week to week however the main variable you need to take into consideration is the lifespan of the components you all ready have.
For example a good gaming personal computer that you buy right now should have all the necessary component required to last you at least one year and it will support all the new programs that will appear and the games that are to follow. The good thing about manufacturers is that they base all their new products on software and characteristics that personal computers all ready have.
No one will base their new component on a platform that has not yet begun. The only component that will not affect you very much will be the hard disk drive that will have the same space no matter what. It is simply a matter of choice and space requirements.
The best recommendation is spend a bit more money than you would, buy only the pieces that have appeared in the past 3 or 4 months and such a configuration should last you around 2 years that is if you want to stay head to head with the big contenders of the market. Pretty much you need to think what type of programs you use, what games you prefer and the exact purpose of your personal computer so that you purchase a configuration based on your own preferences. This way you should consider that buying a personal computer is much like making a long term investment.