Motorcycle gets a change at ~3,000 miles, full synth, but it also revs high, and the oil is also used for the clutch, so I will keep doing it that often, which is normally only once a year.
The two commuter cars get it every 4-5k, both on standard oil, about every 4 months.
The weekend and trip car we have gets it every 5k and it has synth, but it is used less, so it is every ~8 months.
I remember reading a long term test review of the Pontiac G8 GT by Edmunds. It used full synth and the manual said to change the oil at something like 22 or 25k miles and it has an oil life indicator gauge as well. So, the used and abused the car the whole time and before selling it off, they had it changed and sent the oil out for a lab analysis. With 22k on the car and the oil life monitor saying it had about 8% life left, the lab came back and said it had about 10% life left.