My raised beds are all pine (2" x 10" or 12"). The life varies. I get between 5-6 years. But I do use raw linseed oil and put it on very heavy. My three corn beds are now 7 years old but they are rotting. I will use them until they fall apart.

Cedar would definitely last longer but not sure how much longer than pine. I've seen cedar rot within 10 years on a poorly constructed deck. Sealing the wood, regardless of what you use will help. Lindseed oil is a good choice as mentioned by yolos above - I believe heating it will make it soak in better, is that right?

A couple of ideas when you construct them:

- support the corners of the beds with pavers or bricks to keep the wood from direct contact with the ground; if these are long beds, also put some pavers mid-way down the long sides; you can bury the pavers so that you have a minimal gap between the wood and the ground - yes the wood will still be in contact but won't be bearing down completely with all it's weight

- line the insides with 6 mil poly sheeting

- try burning the boards using the Japanese technique of yakisugi (https://en.wikipedia.org/wiki/Yakisugi) - obviously be careful! Never tried it myself but I've been to Japan where they have yakisugi treated wood that has lasted hundreds of years

- avoid having end grain in contact with the soil or exposed to rain from above. I see this mistake made often with posts that tie together the corners. You need to cap any end grain in posts with either a post cap or a trim piece that picture-frames the tops of the beds (handy place to sit on as well).

I did build my new beds with cedar - cost me about $750 Canadian plus another $300 or so for soil to fill them. I have about 224 square feet.

I can't post a photo, so here's a link to the photo on my Instagram account: https://www.instagram.com/p/BdafjqvBwJz/

If you have any gopher activity in the neighborhood, line with hardware cloth!

We built raised beds about 3 years ago and I was in a quandry about which wood to use. We had used pine in the past and got a decent 8 years out of it, but it was rotting from the inside out for the last 3 years. I didn't want to spend what they wanted for cedar and no redwood in our area, but I searched and called so many lumber yards and one in particular that was over a half hour drive from me, said he had been steering people toward using rough sawn Eastern Hemlock, which lasts almost as long as Cedar, but is more economical. That's what we ended up doing and so far so good.

Although treated lumber is less toxic than it used to be, I still don't want to use it for my vegetable beds. However, having built raised beds from a variety of untreated wooden materials over the years, I also got tired of them lasting a few years, rotting out, and/or coming apart over time. I finally decided to try making them out of 1X6 Trex-type synthetic decking materials and coated deck screws. It was pricey, but it's been ten years and they are still as solid as when I built them. The pieces come up to 20' long, which allowed me to build rows of long tomato beds. The only design issue is that they are a little less rigid than regular wood, so I screwed in a 1X2 transversely every 4' to prevent bowing of the long sides.

Pressure treated or tanalised scaffold boards - even second hand, they are good for a decade. Red cedar (thuja plicata in the UK) is far too spendy for veg and flower beds...but unplaned larch is a good substitute.

Here, plain untreated pine will last a decade.

Here, red cedar is Juniperus virginiana. I don't know if it is commercially available. It is usually obtained by means of a chain saw. I've got some red cedar posts in the backyard that have been there for almost 20 years.