How long does it take for trash like beer cans, old mattress springs, or glass bottles to go from trash and need to be packed out to being a historic artifact with archaeological significance?
How long does it take for trash to become an historical artifact in the United States?
There appears to be a legal answer, which is "50 years". It is not clear whether this legal answer applies only to Federal land. The clean-up link takes the reader to a site under the aegis of the National Park Service and which refers to a cache of cans at Big Bend National Park.
Let's assume we are talking only about Federal land. I don't want imply that I take breaking the law lightly, but let's bear in mind that the saying below, made famous by Charles Dickens and dating back to at least 1654, is sometimes true.
We have hiked many times in the area around Desolation Lakes, near Mt. Humphreys, accessed from Piute Pass, near Bishop, California. The area was used extensively by sheepherders, mainly Basques, in the 19th and early 20th centuries. They never heard of LNT, and would have been too busy to follow it if they had.
At least one packer in the area did extensive cleanups of the area in the later 20th century, in the '70s and '80s. It wasn't just that a littered "wilderness" would not be good for his business, he loved that area and wanted it clean -- he said it was his living room. When we found an old sheepherder site he missed, carpeted with old glass and cans, we double bagged the litter -- maybe 20 to 30 pounds of it -- and gave it to him to pack out with our gear. (The packer said the site was probably an old sheepherder site.)
Generations of archaeologists and sociologists will have lost something by not having those glass and cans from 19th century sheepherders to interpret, but generations of hikers and backpackers will have gained. We must each decide what is an artifact and what is rubbish. An Anasazi potsherd is a relic; leave it alone, and report the find. A beer can from 1966 is rubbish. Haul it out.
In summary: There are things we must protect for future generations; beer cans, Styrofoam clamshells and plastic bottles are not among them.