Coyote Works, a desert and wilderness survival channel on YouTube, planted a Ruger 10/22 in the earth about 15 months ago then went back to see if it sprouted.
Buried in a shallow 14-inch deep hole, the rifle and ammo are inside a sealed ZCORR mylar firearms bag, which is inside a black plastic trash bag, and wrapped in a foam blanket. A desiccant pack and some snacks complete the setup.
Overall, the gun, ammo and mags look really good for what he says is 15 months including a hard winter in the earth. So good that we had to go find the original video from when he deposited the plinker, and it is below. Besides the published date of about a year ago, both the surrounding area and the lead farmer look slightly different, so we can buy there is a time gap between the two clips. Still, keep in mind the terrain is dry desert, so your mileage may vary.