I think a good rule is 10,000 miles for synthetic, ~6,000 miles for a synthetic blend, and 3,000 ~ 6,000 miles for a conventional.
I'd recommend changing it every 6 months. I do it in fall and spring, on some vehicles I like to switch from a 5W-30 or 0W-30 in winter to a 5W-30 or 10W-30 in summer. A good rational for the 6 month rule is that, if you aren't driving enough miles to necessitate changing the oil, you probably aren't driving enough miles to heat up the engine to burn off condensation inside the engine. Sitting for long periods or only taking short trips is bad for the oil.
If you're stretching the oil changes out to 6,000-10,000 miles, try to remember to check your oil periodically. Maybe you won't remember to do it monthly, so do it every time you roll 1000 miles and see a bunch of zeros on the odometer. Not changing your oil isn't good, but realistically you could probably go tens of thousands of miles before you did significant damage. Running your oil low will burn up the rings and permanently damage the engine. Keeping it full is the most important thing.
I've got 435,000 miles on my 1997 Plymouth Grand Voyager. The engine was rebuilt at 350,000 miles, I had the crankshaft and rod bearing caps off recently and the bearings looked brand new. It's gotten Castrol 5W-30 every 3,000 miles, my stepdad and I used to change it when I was a kid. Now it gets Castrol Syntec High Mileage, every 3000 miles or 6 months, whichever comes first.