Two times a year if you can. Grime and dirt will build up on the coils, making your fridge work harder and run more often. A special long-handled brush or vacuum attachment will make this easier to get done.
If you have a brand new refrigerator, or have just added an ice maker, the new plastic lines are still leaching out some of the plastic smell. Throw away the first batch or two of ice. If you have a water dispenser on your refrigerator, you may want to run some water through the dispenser and toss that out as well. After a few gallons of water have gone through the system, all of the plastic smell and taste should be gone.