To answer this question I capture 2 MEN from a new monti cap I got from the local fish shop and placed this in a small container with only saltwater. I top off the water in the container once a day to make sure they are not killed by salinity changes.
So far it has been 13 days (2/13 - 2/25) and they seem to be doing fine. Mostly they just sit motionless on the side or bottom of the container, but I also frequently find them floating on the surface of the water (my guess is that this is a behavior they use to get swept away by the current in an effort to find a new food source). If I touch them with something they will crawl way from me so I know they are still alive. I will try to keep updating to help give a definitive answer to how long these pests can survive without montiporas to eat. Let me know if anyone has any suggestions.