My oldest bed is 11 years old and it is starting to rot. All of my beds are made of untreated fir or pine. I only use 2"x12" lumber now as it seems to last the longest for me.
I have tried 1"x6"'s before and they rot in about 5 years. I live in the Pacific NW and get rain approx. 9 months a year and all that rain takes its toll on the lumber. Depending on your climate conditions and how thick your wood is they should last at least 5 years and probably longer.

I used linseed oil on two of them when built but have not noticed any preservation of the wood. Good luck!