In my opinion they were placed there sometime in the early morning hours of the 14th.
I say that because I believe that the area where the girls were found was searched at least once. It was said that a sister dropped them off and was to pick them back up. That sister is very familiar with the area. If she looked for the girls it is highly likely she would have walked one of the paths that heads East from the bridge (we have videos of going that way - one was posted the other day with the deer stand) that eventually ends up at the point above where the girls were found. This would be the path that heads down where RL was showing the media the crime scene (we see his dog in the one photo with the cameraman down near the crime scene tape). If it were still daylight and someone went down that path it would be hard not to see the girls. Especially since there would have been two bodies. One could argue you could miss one - but miss two? I doubt it.
Also, the Fire Fighters that we see out in the WLFI footage from the night of the search are out on the North side of the creek East of the bridge. Because it would be possible for the girls to have drowned or injured near the water and Delphi has a high water rescue team I expect that they would have went through the areas closest to the creek first. They very likely have trained there and are familiar with the area. Any of the other volunteers out that night would have walked the paths - and one of the paths leads you right to where the girls were found - because if you were looking for girls on the move in those woods it would be expected that the girls would also be walking the paths rather than tripping through the trees, underbrush, debris, ruts, and holes off the paths. And I don't see how you could miss two bodies given all that.
I could be wrong.