All my beds r made from scaffolding boards. They came with me from last house, so have been in the ground for about 7 yrs now. My beds are 8'x 4', as the boards were 8' to start with, so just cut some in half. Perfect size, can reach or step over, and not too long to go around when taller plants are in. Have to say that mine are only staked at the corners and I have had no problems. However, where I have used thinner boards as a border, thinking that they had less weight to support, I have needed more stakes.
I did build 16' long beds at the last house, thinking that I would save path space, but it was a pain in the bum walking all the way around them if they were full of big plants and I left a trowel or something at the other side. I've also found that more, smaller beds allow for more crop rotation. (although I do get tempted to grow more and more stuff that we can never eat!) I have 6 beds, since I stole the kids sandpit, a fruit cage of 16x8', a currant bed of 25' long and still plant fruit and veg in any corner or pot I can find, including an old bath. And there are just 3 of us. I only like toms, onions and the fruit! I do manage to keep us in fruit and veg for most of the yr by freezing and preserving, and only really buy in main crop spuds, onions when I run out, and sometimes carrots as they get eaten small and never make it to a maincrop. This yr, I direct sowed my broccoli and cabbage, then discovered I have clubroot, but the caulis did fantastically well, I think cos I stated them off inpots as was short of space. So we have lots of cauli, but will have to buy broccoli. (what a waste of time and space, won't make that mistake again!) And I still have mange tout and beans growing well, sick of sight of 'em, now! And a late crop of baby carrots, lettuce, turnips and beetroot, and Christmas spuds just covered with fleece now in the bath, along with loads of raspberries! Garlic and Autmn onions from seed to plant out tomorrow, then start my early peas and broad beans in coldframe. And my blueberries came yesterday!
Raised beds from boards can be very productive as you can easily cover them with black plastic to warm them, fleece to protect early and late crops, mesh to keep out the bugs. The wood insulates the soil to a degree, they drain well as you can control the soil so much better by adding anything necessary, Feeding is more precise so you can crop more closely. Just so much easier to grow more of everything.
Sorry, I can't shut up! You only wanted to know how long they last! I reckon I'll get at least another 7 yrs out of them. There are no signs of rot, yet, and I only paint the outsides with waterbased shed and fence paint (Cuprinol or sim) to keep them looking neat every couple of yrs.