Over the years I've been playing with small 4-stroke engines I learned these facts:
1. Change the oil often.I change the oil in my engines after every 15 hours of running. I run my engines hard and in a hot,dusty climate. Use a quality oil. Oil's that fight the build up of sludge are the best.
2. Keep the air filter clean. Running it after it has turned color and is clogged will cause the engine to run too rich and will not idle well. Foam filters can be cleaned. I just do it when I change the engines oil.
3. As stated by several others, check your spark plug electrode color. This will tell you how your engine is running.
4. Install a fuel filter. There are many small holes in the carb that can become clogged. you can get a bag of these disposable filters off of Ebay or OldMiniBikes Warehouse cheap.
5. Use good gas. Keep the inside of the tank clean. Drain it over winter! Do the carb also.
Some folks use Sea Foam as a fuel system additive. It does work.
6. At least once a year check the valve lash ( this will help you determine how you oil is working and see if the burn is too hot/cold of the fuel charge.)
7. Check all bolts and fasteners for proper torque.
How long your engine will last is up to you. If you maintain it and keep it running as designed, it will last for many years. I'm running a B&S engine now that I have had since 1968. All I have done is simple maintenance over the years. I HAD a very nice Tecumseh 2.5 HP engine on my Bird mini bike until I got stupid and tried to hop it up. It HAD been a faithful runner for over 8 years and had pulled me all over Maryland for 12,000 miles before I blew the engine. While this sounds like a lot of miles, it really was not. I lived ten miles from school(s) and my house was next to a very large forest.I rode that mini enough I broke the weld on the steering yoke! wish I had that bike now...