Depending on the model, BUFFs have had significant refitting of structural components. Some D models were re-skinned on the lower wings and parts of the fuselage in the 1970s (the Ds were retired in '83) and the Gs and Hs had wing spar replacements in the late '70s, as low-level flying was taking a toll (the Gs were retired in '93).

Overall, all military aircraft are subjected to significant testing, both at the airframe level and component level for some things (like landing gear), with replacement as necessary.

Basically to get unlimited fatigue life with aluminum parts you need to keep the cyclic loading below 5% of ultimate.

So all the aluminum will eventually need to be replaced.

I don't know what the 'safe life' or a B52 is, but they estimate a number of hours/cycles for crack growth to begin and base maximums around that.

The air force does damage tolerance analysis too (for life after crack formation), but I think that started after the b52 program (and i cant remember details offhand)

I guess a combination, they aren't being used that much so wear and tear is minimal probably