By limit, do you mean the cycle value at the transition between high cycle fatigue (HCF) and low cycle fatigue (LCF) where the stresslife method (SN) becomes invalid? If so, then there is no precise cycle limit. In theory SN is only valid for stresses below yield which results equates to a allowable cycles of about 10^3 for many ductile metals. So as a "rule of thumb", 10^3 is often considered to be the transition from HCF to LCF. However, the true value varies based on the material and some SN curves extend below yield, so SN is often used even if the stress is slightly above yield.
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