-       get in and out of bed by yourself

-       bend your knee to a 90 degree angle (or are making progess towards this goal)

-       straighten your leg completely

-       climb 2-3 stairs and walk with crutches or a walker

-       do the prescribed home exercises

Your incision site should also show no signs of infection, and your pain should be under control enough that you can successfully complete your rehabilitation exercises.

Most knee replacements last between 10 and 25 years, depending on the type of replacement, your activity level, and other factors.  Some doctors even ask that you come in yearly to have the replacement checked for damage, since some devices can be repaired if they are showing signs of wear.

Sources: 

Arthritis Care & Research

Dr. Jonathan Cluett, About.com Orthopedics

American Academy of Orthopaedic Surgeons

Mr. Charlie Davis, Knee Replacement Patient

PediatricOrthopedics.com

Wikipedia.com

Total Knee Web