When it starts leaking or making noise. I'm not aware of any kind of recommended replacement interval like there is for timing belts or oil changes.
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=== Update (03/13/25) ===
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No problem, Kody. I don't know the details of your rig yet, but on my son's 1985 GMC High Sierra (K2500) with a 350 ci engine, it's a pretty straight-forward job. Once you have the belts out of the way it shouldn't take more than a half hour or so; add another half hour for the belt R&R and an hour should be enough time.