How long does anyone have the right to leave their property on public land?
I don't know the answer to that. Perhaps it varies by state, or by county. If there is a law that prevents someone from posting something on public property, or has a time limit on it, one would think that this falls into the jurisdiction of the local authorities. If those crosses that were posted were illegally on public property, the local authorities would have, or should have, dealt with it.