The vine mine cast by Elite Skull Crusher giants seems to be nearly permananent. I looked duration up in lore and it says 1 turn per level. I think they are about 23rd level casters, so I made sure I waited half an hour to try to get a grave back which was surrounded by one of these vine mines. The vine mine was still there, keeping me from getting my grave in greater sanctuary.
Aside from my current, personal, issue with the vine mine, I suspect that these vine mines are a source of lag. I've come across old vine mines from other peoples fights a number of times on Xantril. It seems like this spell could actually last a round per level and not cause any problems. It is a combat spell and, in my experience, if you can't win a fight in rounds per level, you are in serious trouble. : )
Thanks,
Esi