Sjanky says:

It's a nice marker between 100 and 500 miles.
And it's not 250, because that would be too easy.
I started Aug. 24, 2006 and reached 500K July 25th, 2007.
Within 11 months (o.k. + 1 day)

 Morgan says:

It's also a nice marker, half-way to 1,000km

I'm not quite there yet, but I started at the end of 2006, using the Nike+ gadget on almost every run...