There is no list of "expendable" parts, nor life expectancies. No parts should wear out in any particular time. Well-kept engines can easily see 100 hours or more of use. Corrosion, bad fuel, and lean running are the most common causes of shortened engine life. If you run your engines conservatively, make sure they don't get any corrosion, and they are kept clean, you'll find that they give years of reliable service.