On my 2006/2007 Honda Odysseys:
After 1k miles, oil is very light brown (looks nearly brand new).
After 2k miles, oil is between light and medium brown.
After 3k miles, oil is medium brown.
After 4k miles, oil is dark brown.
I change the oil at 4k miles or 6 months, which ever comes first.
This covers the short trip vehicle which always has less than 4k miles when I change it at the mandatory 6 month interval.
I only use High Mileage Full Synthetic oil with a higher HTHS than the viscosity specified in the owners manual
so that under heavy loads / high RPM there is additional MOFT for additional engine wear protection.
Two are my goals are to prevent sludge, and also prevent engine seals from leaking internally causing oil consumption, or leaking externally causing leaks on my driveway/garage. So far I've been fortunate, no oil leaks at all. No oil added in between oil changes.