Ive seen plugs last 25k+ miles.
However, at $8 for all 4 of them (Champions) if they Look bad or I Feel like it, I will replace them. The cost just isn't high enough for me to penny-pinch on.
Some people clean them too.. This is another thing I tend not to do unless diagnosing.. then if I clean them and it fixes the problem, I replace all 4.
There is nothing wrong with cleaning them.. I probably would if the plugs cost $10 each, ya know? But at $2... nah