Have visited/reviewed for repurposing 3 vintage Dutch Barns over just the last few months all over 200 years old...
Many of these have section exposed, and because they can "dry out" there is little appreciable damage occurring.
If your frame isn't oiled (aka bar wood) you could probably expect 2 years of safe exposure before worrying terribly about structural degradation of being an appreciable concern. Cosmetic change in affect is another matter, and open to personal interpretation as regards to aesthetics in wood coloration.
If the frame is oiled and the joints waxed well before assembly that time frame could stretch as far a decade in some cases before great harm befalls the frame in my experience.