It is not such thing like a normal level of fatigue.
Fatigue is a physical or mental weariness resulting from exertion. In this case you use more energy you should and this lead to fatigue. Also this energy can be drain by stress or intellectual work, not only by physical exercise, physical work etc.
Converting it on human language – your body is begging for more energy. You should have a decent rest to restore energy buffer.
So, fatigue is a condition you can compare to fuel reserve in your car. You can drive on reserve to nearest gas station. What is the normal level of reserve? In normal operation you should have fuel level above the reserve. It is the same with energy your body should have.
You should check your health with thorough medical exams if you have any level of fatigue. If everything is OK than it is another reason for it.
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