That's out in the desert and actually the barrel is in pretty good shape, but I've seen stuff near mining camps dating back to 1900-1930 with pretty much the same patina, leaning up against the tree trunk. Tho I did find an model-t axle embedded in the crotch of a tree once. It was about as rusty as an anchor chain left out on the waterfront for a few decades, ie almost completely converted to rust. But I've found stuff part buried by accumulated detritus with negligible patina, obviously untouched for decades. Just depends on the locale and particularly precipitation.
When we went camping out west in the 60s-70s we also found a lot of tin cans high up in the trees. My mom said it was once the custom was to use tin cans to boil water and just set the cans in the tree for the next camper to use. If the tree didn't get much water or sun the cans would still be at eye level after 30-60 years, but if it was relatively wet you might spot the remains of the can in the treetops.
In Nevada it mostly depends on the orientation of the piece. If it is on a North slope it will rust faster than one on a Southern slope. A guy I work with found a similar Winchester in the far Northern part of Nevada about 15 years ago. It was on an East facing slope and not in as good a shape as the one in the OP.