How often does a semi truck change its oil?

5000 to 10000 miles

In reality, it depends on the trucking company. There are two things a driver can do to stretch out her oil life--keep control of her idling and run her engine at low RPMs. If she idles the engine all night to run her air conditioner so she can sleep comfortably, and if she runs the engine at high RPMs, she will crud up her oil and shorten its life. If she has an intelligent idle controller that starts and shuts down the engine only when it's needed to warm the cab up (most new trucks have this), or she has an APU so she doesn't have to idle her engine at all, she'll get a lot more life out of her oil.

As for lowering your cruising RPM, if you order your rear-end gears so you can drive 55mph at 1200rpm you will not only make life easier on your oil but you'll save somewhere between 1/2 and 1 mpg--which doesn't sound like much but a truck that runs 12,000 miles per month (some trucks will go much farther every month) will burn 2000 gallons per month at 6mpg or 1846 gallons per month at 6.5mpg.

There's one other thing to consider: oil analysis. It is possible to run chemical tests on oil to decide when to change it, which will also tell you when bad things are about to happen to your engine. If your oil has metal off the bearings in it, you know you're wearing out the bottom end of your engine. A heavy-duty truck engine costs more than a new car, so you really want to keep close watch over it.