Toyota recommends 7500 mi between oil changes. Some fear that this is too long. To reassure everyone, I took a picture of my oil at 7600 mi, just before I took the car in for my first free servicing.

The top dipstick is from my Prius. The oil is thin and light tan,
but there are a noticeable amount of suspended particles.

The bottom is a screwdriver dipped into oil I drained out of my
previous car at 3000 mi. It is thick, sludgy, and opaque brown.

Clearly, it was time to change the oil. But more importantly, the Prius' oil was much fresher at 7600 mi than a "normal" engine's oil at 3000 mi.