This is just straying way off topic.
PM schedules look at the probability of failure, the cost to repair, and the cost to not repair (direct and indirect cost of failure) plus the cost to repair. Predicting the failure time on the serpentine belt tensioner is pretty interesting, since the service the engine has seen varies so much. It's probably best done based on amount of fuel consumed, as that's related to time, engine speed and load. Still nothing for environments conditions like heat and dirt. But nobody really tracks that on an accumulating basis, at least well enough to get a good bit of failure data to analyze.
As far as cost-to-repair, simple parts $ plus labor $. Labor is pretty much the same (plus three bolts) for ne rollers with belt vs. just the new belt. The cost-to-not-repair adds the inconvenience factor with down time, may include towing and possibly lodging, and also includes the parts and labor cost since you'll need to get it fixed anyway.
I pretty much DIY all this stuff, shop for PM and maintenance parts based on value, and get to schedule the work at relatively convenient times, in my own heated and air conditioned workbay, with my own tools and easy access to more if needed. In my experience, failures very seldom happen at home in the workbay, just as I might be headed to the parts store anyway, on the day I'd already scheduled for exactly that particular work. I'm just not that good at "just in time" maintenance and repair planning. The Great and Wise man Edsel Murphy does the majority of my 'run-to-fail' scheduling for me. That translates to the side of some lonely highway, out of cellphone coverage, rain, heat or snow, and a noise generator riding along in the car. Or worse but even more likely, it's that same noise-maker stuck in that same place under the same conditions.
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Is there a reason to inspect and service the alternator prophylactically? Why yes, there certainly is! Easy enough to spin the alternator by hand and feel for bearing grumbles, and listen for bearing noises while/whenever the belt is off. Replacement bearings are amazingly cheap to replace at that point. While you are in there for the bearings, you'll install new brushes, clean and dry the windings and the case, verify the overrunning clutch operation. Is a full remanufactured alternator a better choice? Might be for some. But bearings ALWAYS offer failure symptoms before brushes, so using those symptoms as a flag isn't a bad idea.
Alternators see electrical abuse when they are tasked with charging a tired battery. They also suffer when there are poor connections in that charging loop. Don't abuse the battery by running it down, and you'll have less alternator abuse while recovering it. Keep the connections and cables clean and protected. Test the battery at least annually, and replace it when it shows signs of loss of capacity. On some of the cars, the alternators get new bearings every ten years or 100k, because the likelihood of bearing failure goes up as the grease in the bearings starts to gel. Same grease that's in the belt tensioner rollers, BTW.
Use the knowledge and experience of those who have passed this way before you. Education like that is way cheaper than experience with failures as a teacher. I read posts here from others, evaluate my own car and situation to help decide if I should plan for some PM so whatever happened to you won't happen to me. I do a LOT of PM on the cars here, but very seldom do any real "work" and almost never end up "fixing" anything that's broken. I'm also quite comfortable knowing I jump into any of the cars, newest or oldest, and do a trip to a far corner of the country and back with clubs and luggage, and not worry a lick about the car itself making the trip painlessly. Not the driving part anyway; putting is a separate issue.