Turbines in jet engines
Both, and it depends.  When a Hot Section Inspection is done, they are looking for mostly blade erosion beyond dimensional limits, and cracks.  Some cracks are allowed if the OEM's engineering determines they are benign (sometimes cracks form which fully relieve the stress that initiated them and the crack stops propagating - such cracks can be left alone, and the engine OEM will have clearly defined crack allowances and limits).

Erosion is mostly a function of how dirty the atmosphere is.  An engine running in the high Arctic that sees very little dust (outdoor dust close to the ground is mostly silica particles - sand paper without the paper) may run a very long time before significant erosion and the hot section might only have to be rebuilt due to burner can or blade cracking from operating cycles.  

An engine running in the Middle East may only run half the normal TBO before blade erosion forces the engine off, made worse if an operator avoids using flex thrust to minimize engine demands on takeoff.

Then, some engines are more susceptible to erosion than others, depending on the compressor inlet design.  Turbofans with a stronger ram effect (good for overall efficiency and for windmilling after a flame out) suffer more than those with less.