I realize you're more concerned with time than mileage in this thread, but FWIW I just added this post to my blog today which you might find interesting. It includes all the oil analysis results from my oil changes, which I do every 10,000 miles (~16,000 km), which for this latest change, happened 6.5 months apart.
http://www.seriesblueadventures.com/...-oil-analysis/