When you remove the old worn BB is it spinning roughly? Does it feel like the lube is gone (spins smooth but with out the buttery feeling)? Before you take out the BB is it different feeling? How much rain does it see (hard to quantify)? How big are you? how much off the saddle pedalling do you do?

The life span is a hard answer to give you. By my assumptions you are riding, maybe, 5000 miles a year. If the above factors are all in the "wrong" directions then only getting 2k miles on a BB might be all you can expect. But if the factors are all in the less stress/exposure direction then a BB a year would be more likely. One of the questions i have is whether the wear is from stress (weight), cycles of use (# of miles), exposure (rain/mud) or some not yet discribed.

There can be issues going on that make it seem like the BB is worn out. Like it's loose in the shell or crank arms are not tight on the axle. But one would think that if you've been replacing the BB so often that you've learned how to check these things out and stay on top of them.

The only way to really answer your question is for you to up the anty and shell out for a high end BB. Like a Phil. Then see what happens. And do let us know what you find out. Andy.