You can extend the life of the wood by using a waterproof membrane on any surfaces that come into ground contact or you can use a waterproof sealer, of which there are a number of environmentally friendly options.

Or you can build a raised bed from CMU's/concrete blocks available at any box store for a couple of bucks each and your bed will last as long as you need it to!!

I'd go with treated lumber. It's not much more expensive than pine but it's cheaper than cedar. Today's treated lumber isn't the toxic stuff it used to be (the use of arsenic was phased out in the early 2000's). If you want to go with pine then the timeframe you give is about accurate.

Rodney