In continuous use, they will last until you break them, the wood will not deteriorate unless you get a bad infestation of wax moths, which burrow into the wood to make coccoons.
Supers should last decades if you keep them painted, we have a number of brood boxes and supers that are at least ten years old and are just now needing new paint, otherwise they are fine.
Keep the joints waterproof and they should never rot -- the bees coat any exposed surfaces with propolis. They can't seal the fingers, so I use Titebond III there.
Make sure your finger joints are tight -- if you need to tap them together that's perfect. Less exposed surface to get and stay wet.
I've been using boiled linseed oil followed by good alkyd primer and top quality "oops" mistint paint, two or three coats. Those boxes are fine, some of the earlier ones I only used alkyd primer and exterior paint on are peeling due to poor adhesion of the primer.
A nice close fit between boxes is good too, but you are going to damage the edges prying them apart, so don't worry about perfect. The bees seal them up pretty well with propolis, but a good fit results in boxes easier to take apart since they have less "glue" between them.