infininte....I have had my boxes for 12 years-the original ones. But, mine were treated. Materials used for paint these days is a lot different than it was 15 years ago. Every couple of years I put some more color on them, but I only paint the outsides of the boxes. I think you could get a look if you wanted to visit my site...there should be pictures in there somewhere. Its still a work in progress though so be patient...its been really, really hard for me to learn that stuff...

I think people in cold climates get many more years out of wood than those of us down south.

4'x8' beds

Standard concrete blocks in a single row can work well where you only need an 8" depth with two small downsides; their 8" width takes up a significant growing area, or if you build the beds wider it gets harder to reach the middle. Blocks two rows high lack a desired stability.

For only a few dollars more you can have poured concrete 3.5 inches wide and 16" high. Material for reusable forms makes the first box more expensive, about $80-100 (14 80lb bags of premix concrete plus 60lf of #3 rebar plus forms). The result is a deep box that's permanent, strong, non-toxic, with a full soil bed width in a narrow footprint. The downside is the labor to make the forms and mixing concrete is harder work than some care for.

pls8xx I agree.
Jonhughs is a Gardenweb member.

Here is a link that might be useful: jonhughs is a great guy & gardener!

Last year I bought treated 2x10x8s and cut one in half for the ends to make a 4x8 bed. I lined it with plastic even though today's treated wood is "supposed" to be safe.
I also attached a piece of treated decking on each side for a seat or kneeling place while working.
Very cost efficient.
I did till into the ground soil about 4 inches to add depth to the bed, making it about 14 inches deep, and added compost and soil conditioner to loosen the mix, then added the frame and added more compost etc. until the 4x8 frame was full.

first year i started it, i used regular 2x8s. after 2 seasons, the insides were eaten away by beetle larva.

the next one i built out of trex. the plastic/wood lumber at the box stores. expensive, but already stained and last forever. ended up going a little bigger, 4x5 (i have a small yard) and used the same stuff(gave the other one to my neighbor, easy to disassemble and move, still in use).

assembly was 2 6in boards high. theyre a little flimsy so i used strong ties in the corners and the sides to hold everything steady.
i screw little eyelets at the 12in mark along the sides and thread jute twine to make the grids. at the end of the year i cut it and leave in the soil.

thus far ive had zero problems. admittedly it WAS pricy at first, BUT it lasts forever, looks pretty effin good, nothing eats it(last i looked its considerably cheaper now than when i built mine, so take a look)

Right now I have spruce boxes going into their 4th year and in great shape but I'm in Maine. I am slowly buying cinder blocks to replace my beds in the future when they start to fall apart. I expect to use the open spaces in the cinder blocks to plant small crops, like rads, lettuce, carrots, etc so as not to lose the growing room and even adding to it.
Treated wood is still iffy in my book. Chemicals that can seep into your crops aren't my idea of healthy. To be safe, I'd stay away from them but that's me.