Fatigue life? I suspect we might run out of AVTUR first...
Here is the data for the KC-135R from which ours will be based. This should be worst case.
The March 2004 Defense Science Board Task Force Report on Aerial Refueling Requirements found that "Usage, which induces material fatigue, is not the driving problem. Total flying hours are relatively low for the KC-135s: the current airframe average is about 17,000 hours. Fatigue life is estimated to be 36,000 hours for the E, 39,000 hours for the R. Cycles are commensurately low on average (3800 for the R and 4500 for the E). Thus, the airframes should be capable to the year 2040 based on current usage rates."
Source
KC-135R Stratotanker