How long an engine lasts requires a long list of variables to consider before a time frame or mileage can be given. I have seen engines last less than 40,000 miles and others that lasted more than 300,000 miles. When it comes to the time an engine will last you, look at mileage per year or how the vehicle is stored if there is very low mileage on the engine. Think of a 1916 car engine that is driven only 100 miles a year and put in storage after each use. After 100 years the engine will only have 10,000 miles on it. This motor may last a few hundred years before it would need rebuilding or replacement. This is why we use mileage to rate how long an engine will last.
Most engines on the road today are designed to last well over 100,000 miles. An engine will last much longer if the engine is not abused in any way and all maintenance that the manufacturer recommends is done on or before it is due. All repairs should done as soon as possible without damaging the engine by waiting to get repairs done, like overheating the engine from a failing water pump. If you take great care of the engine then it will last a long time for most manufacturers. Some manufacturers of engines made their engines from lower quality parts or materials and these engines may not last that long even if they are taken care of well.
Abusing the engine, no matter its quality, can and most likely will shorten the life of the motor. When you do not change the fluids and filters when recommended by the manufacturer then you will cause excessive part and component wear on parts like bearings, pistons, valves, or timing chains and gears. If the engine is overheated and driven till the engine stops because of ignoring the temperature gauge or warning lights, it can cause irreversible internal engine wear or damage. If the engine is used for towing all the time, like pulling a trailer everyday or hauling extra weight all the time, your engine works much harder than usual. It’s like you walking ten miles every day wearing a heavy backpack versus not having the backpack on. You will get more tired and worn out with the extra load on you and over time may get problems with your body that you may not have gotten had you not been carrying the extra load. The same holds true with the engine. If it does less work it will last longer and require less maintenance and repairs.