There are no hard and fast rules.
Only general guidelines driven by moderator experience and gut feelings.
Often a mod who is worried about a thread will discuss it with the rest of staff to try to arrive at a consensus of whether the thread should be closed, disappeared, whatever.
The only truly sure thing is that spam messages will always disappear. This is a firearms board, not the Nigerian National Cellphone Vendorama, and Rich doesn't pony up his bandwidth so that some shyster can make a buck.
If you're feeling as if you're seeing messages one minute, and the next minute they're gone?
It's the blue pills, man. Either cut the dosage, or up it, but don't try to maintain.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.