As always happens, it depends...

If your Design Allowable Stress was established based upon 100000 h averaged rupture stress, then 100000 h is the Design Lifetime. You should be aware that Design Lifetime correspond to effective operating service conditions (e.g. continuous work under design loadings action), and it is not a calendaristic period. All the shut-down periods are excluded.
However, if the System was operated for a longer period than 100000h, then you need to adjust/de-rate the allowable stress, by decreasing the averaged rupture stress in accordance with the effective operating lifetime (example: 130000 h > 100000 h). Such assessment is generally done using Larson-Miller parameter (B31.3 Appendix V)

Then, you need to clarify/define which is the Operating Service regime. What kind of pressure & temperature loading/unloading cycles are applied to your system. Is it one single set of P & T parameters, or you are dealing with several cyclic loads (example: reaction and regeneration regimes, each of regimes defined by an Operating Temperature and an Operating Pressure).

In addition, you need to clarify which are the frequencies and time-durations of these cyclic actions? In general, for simplicity, transient portions are conservatively included in full-loading periods.

Based on the answers to the questions raised above, you'll be able to estimate the effective numbers of loading/unloading cycles. For several cyclic loads, cumulative damage principle (Palmgrin-Miner) may be used to establish the equivalent no. of cycles.

Then, using the screening criteria provided by ASME VIII-2 Part 5, or API 579-1/ASME FFS-1, you'll be able to decide if fatigue check is mandatory or not.
If Fatigue assessment is required, then, as pointed out by Michael, you'll need to check the overall/resultant cumulative damage ratio, by superposing Creep damage mechanism and Fatigue damage mechanism.

I suggest to have a look on ASME VIII-2 Part 5, and/or API 579-1/ASME FFS-1, Parts 10 (Creep) and 14 (Fatigue). The methodologies provided by these Codes are equivalent.

Good Luck!