I might as well chime in, since this is my area of expertise (my job).
Start by reading the Owner's Manual and/or the Maintenance Schedule that came with your vehicle. Pay attention the the "Severe Service" schedule. If your driving conditions match it, you need a more frequent service interval.
Use the viscosity and API classification recommended by the manufacturer.
Most modern vehicles call for an interval that is well over 5,000 miles. Some luxury cars are now at once a year. However, the tire manufacturer usually recommends that you rotate your tires every 5,000 miles. I find it most convenient to change my oil every 5,000 miles and have the tires rotated at the same time, instead of dealing with 7,500 oil changes and 5,000 mile tire rotations. If I followed the manual I would have this to deal with.
5,000 mi = tire rotation
7,500 mi = oil change
10,00 mi = tire rotation
15,000 mi = tire rotation and oil change
That means dealing with maintenance 4 times, versus 3 times if I do 5,000 mile oil changes.
Now if you are driving something like a Mercedes or Corvette, it is more expensive because they recommend synthetic oil.
Some vehicles like Bob's use a Engine Oil Life System, where the computer looks at all kinds of driving parameters and then decides when the optimum time is to change the oil. Usually there is a read out on the dash that tells you the percentage of engine oil life left, and then turns on a light as you get close to 0%. Again, with this system it is not taking into consideration that the tires need to be rotated every 5,000 miles. If you opt for the 5,000 mile interval, then make sure the technician re-sets the EOLS computer. If he doesn't know how or does not have the proper equipment to do it, you took your vehicle to the wrong place!
Keep in mind that engine oil accomplishes 3 things: cleans, lubricates and cools. If you stick to the manufacturers recommendations and keep everything else maintained, it is not unusual to get 250,000 miles out of a modern engine.
When in doubt, more often is better than too long. Most modern cars are fine with 5,000 miles unless your severe service parameters call for something more frequent.