Figure on getting around 5 years of use out of your garden beds if you choose to go with untreated wood. You’ll start to see some deterioration and rotting over the years but, you should be able to get at least that long out of them.
There is another option when it comes to using pine wood for raised beds that I want to touch base on. You can always paint it, stain it and/or seal it. Any of these options will help improve durability of the wood and make it less likely to rot.
If economics are the primary inspiration for looking at pine versus a naturally more rot resistant wood like cedar, there are some other things you might consider for keeping your beds economical to build while using materials that are naturally rot resistant.