You can't answer your question, there are so many factors involved. I can guarantee some will see 250k+ miles while some will have given up after a few hundred, that's the chance you take.
What you can say is that they are as good as any other engine out there so you should probably not worry too much about it!
Most 1.7 DTi's don't loosen up fully and give their best MPG until after 100k miles. My Astravan once managed 720 miles to a tank which works out at 62MPG. Not bad considering it was a Y Reg car, plus I was running it on 50% ish Bio-Diesel *whilstles* so it wasn't running at 100% efficiency... Great engine I'd love another.