I got a Raleigh C500 hybrid with a little over 18,000 miles and still have the original BB, not sure the specs of it (but I do know it's a sealed bearing assy). I do remember hearing creaking noises, but not sure when it happened, but never seemed to last that long. And sometimes it would happen during certain times, such as climbing. However, and this is a interesting coincident, just yesterday after my ride I noticed a friction-type noise coming (but it doesn't cause resistance) from my BB, I think it's getting close to needing replacement.

I wouldn't go replacing yours unless there's a some friction involved.