Varies. High-quality square taper or octalink BBs with internal bearings lasted almost indefinitely. I have about 15k miles on my 5500 BB and it's totally fine. OTOH crappy internal BBs (some ISIS models) would let go after only a few thousand. External BBs, from what I've seen here, tend to last less, especially when used in poor weather. Fortunately they are pretty cheap to replace.