[ericthered]

The natural duration of words carved in stone is centuries, which would imply a -15 or so. But on the other hand, They could be taken down shortly, and give humans a lot more agency than otherwise -- if the people involved know about it. If you can hide the stone, or otherwise take measures to prevent destruction of the words, its 15 to 13.

If you've got to leave the stone out in the open or nearby the target or some other appropriate limitation, then it will drop quite a bit to 7 or so: Prominent writing declaring the solution to a curse is to destroy it tends not to last. Of course, if its a binding on a monster, someone could set a watch around it to ensure the beast remain asleep for all time.