How long will Douglas fir planks last if left untreated outside, not directly exposed to weather, but some moisture occaisionally

I have to replace some bad plywood underneath a roof overhang. I have some Douglas pine planks left over from another project, that I like to use.

The (painted) plywood currently in place lasted 55 years, but is starting to fall apart in some places.
It was actually directly in view until about 20 years ago, when it was covered with vinyl siding.
(Previous owner of the house decided to cover it with siding as he didn’t want to re-paint it every couple of years.)
The siding is stapled to the plywood and because of the bad plywood the staples are coming loose.
The spots were the plywood is bad are all due to water damage. Occasional overflow of a gutter, caused some water to penetrate inside the overhang of the roof and get into the plywood.
This happens 2 or 3 times a year when we have excessive rainfall that the gutters can’t cope with.
Otherwise, underneath the overhang, weather won’t reach it. Just regular humidity which is typically 40-70% here.
The plywood lasted quite long in that same spot, so I’m thinking the exposure can’t be that bad.

As I have not much time to do the job (need to rent a boom-lift to work on it, which gets very expensive, real quick) I’m looking for a short-cut.

My idea is to take the siding of, rip out the plywood, replace with the Douglas planks and simply put the siding back up.
As the Douglas planks are not in view I don’t need to paint them, which saves a lot of time.

Obviously I will seal the ingress point of the water from the gutters as best as possible, but I can’t 100% prevent some of it getting to the wood eventually. There is plenty of wind around the house so if things get wet it will dry quickly.

Can I expect Douglas fir planks used like this to last 20 to 30 years?
I’m not worried about the looks of it. Just the structural integrity of the planks.