Expected Lifetime Mileage: Bottom Bracket and Headset
Headsets often last the life of the bicycle. They can get "brinneled" or damaged in a crash or something, but 1500 miles is absurd. 

As for a bottom bracket, on the low end, maybe 5000 miles. The basic Shimano one most people use (UN51/55/similar) can reasonably expect 10k+ miles. But sometimes you get unlucky and get a bottom bracket that lasts maybe 500 miles. You may have just gotten a lemon.