If you use enough linseed oil and it will last forever. I've done some work on a tall ship here in Philadephia (the Gazela) and the masts are covered in the oil. They don't use it often, but the masts are continuosly exposed. Note that they use numerous coats and really slather it on. With any oil you need to add it often and really keep the wood saturated. Also, I've found that the only thing to use for any wooden paddle that your hands will touch if linseed oil, it just feels good, like the finish on those carbon fiber paddle shafts. Yuo can probably use anything on the blades, though. Oil doesn't keep the wood protected from ding, and varnish and epoxy work as "sacrificial finishes." Use the ideas other people have said for that part.