Never.

I use my equipment up as it lives in an urban environment. I consider theft/loss/destruction part of the average lifespan of a product.

I am "green" though and try to buy used items (computers, cars, houses, furnishings to restore) and avoid buying altogether when I can. Thus if I have a part failure (wheel/etc...), I recycle it and just get a used one. I also pay extra to ensure that it's disposed of properly.

Thus, I would run the wheels with no maintenance, as my time is valuable.