Oops, I was wrong about the mileage. From the Mercedes website:

Award Milestones
155,000 miles (250,000 kilometers)
312,000 miles (500,000 kilometers )
466,000 miles (750,000 kilometers )
625,000 miles (1,000,000 kilometers)
1,000,000 miles (1,610,000 kilometers)

So I should hit 312,000 miles (500,000 km) by the end of the year.