Years for normal person XC/trail use as long as you're not riding in a lot of crap conditions. I replaced mine after ~3 years in both bikes and it was probably a waste of time. The bike industry being the bike industry, some bearings sizes can be hard to find from traditional outlets.
And as far as that Revel Ranger diagram posted above, yes, it looks daunting. Reality isn't that bad. Buy a full press set from Amazon (well under $100 now), a couple of old rags to protect the frame, maybe a cement anchor-type screw if you need to do any blind bearing pulling, your favorite beverage and budget a few hours. I have the below and it's $44 right now...
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