Properly maintained, hardwood floors can last a lifetime. The key to preserving your investment is assuring that the finish on your floor is still doing its job. You can test this by going to a high-traffic area where the finish is likely to be most worn, pour a tablespoon or two of water onto the floor. Does the water:
Bead up so that you can wipe it up without a trace? Congratulations! Your floor's finish is doing its job!
Soak into the wood after a few minutes, darkening the floor only slightly? Don't panic, your floor's finish is only partially worn. Immediate refinishing probably isn't necessary, but keep a close eye on the floor.
Immediately disappear into the wood leaving a dark spot? If so your floor's finish is seriously worn and it's probably time to recoat or refinish.