Why replace a unit that is at the correct temperatures? Dont be fooled into believing your current unit isnt better than any new model, that is built to break down. You woulndt be the first person to think there is a problem just because they believe it never shuts off. But honestly, if your refrigerator was running constantly, the food in the fresh food section would also either be frozen, or not cold enough. There is a possibility that you have a warm air leak, and that is causing it to run longer. check your door gaskets or seals for any gaps when closed, or rips, tears or malformations. Also check your door light switches, make sure the lights turn off when switch is depressed. Furthermore, have you ever noticed any fluctuation in temperatures, sometimes colder or warmer than normal. check the condenser coil is free from any dust or pet hair clogs, preventing cooling air from getting to the compressor. Also, make sure the fan near the compressor is running. If none of these seem to be a problem, you might consider a small house call fee and call for a refrigerator repairman to at least inspect your fridge.