I remember seeing a Youtube video of a VW that went 20,000 miles on a single oil change. Looked awful when they drained it. Filter was completely gummed up.
A few BITOG users have gone 40,000 miles, I think they were running the super long life Amsoil oil and fllters, and they did top off with a quart or two in that period. Didn't have any problems.
2 days ago I helped a hot girl I know, she said her dash said "oil low/shut off engine." (She has a VW CC model) I thought, "It has a 'shut off engine' light? Uh Oh!" She said the light was only lit for a moment then turned off. Still, that's not good. The dipstick did not have any oil on it and smelled burned. I took her to get a quart of the 0w40 euro spec that the engine requires, and she scheduled a service appointment for this week. I asked her when the service is due and she said "now, I guess." We both laughed.
Personally the longest I have gone on oil is nearly 8,000 miles on my Tacoma 4-cylinder which has a capacity of 5.5 quarts (running Mobil 1 EP 5w30). Toyota recommends 5,000 miles on conventional, but I drive it gently and it gets up to full temp all the time so I've chosen to go longer on synthetic oil and quality filters.