I have one original Sugino BB on my touring bike, which is 12 years old, and likely has 10,000 miles on it (original bearings). With regular maintenance, they will last nearly forever.
For a sealed BB, I have gotten as little as under 1000 miles on a super-fragile Dura Ace BB, to as much as (I'm guessing, here) 5000 miles on an Ultegra. Even my last "el-cheapo" BB, on a $300 hybrid, probably lasted 3000 miles, over 6 or 7 years.