Any time you are significantly fatter than the rest of the scores list you are in danger of being landgrabbed. Really you are always in danger of being landgrabbed. This also varies by server. Alliance is the worst in terms of danger of being landgrabbed. Tourney seems much more relaxed in that respect, which is not to say that landgrabs don't happen. Both your SPAL and your TDP (total defense points) play a roll in deterring would be invaders, so increasing either of those, your networth can help with this situation. While being grabbed or even farmed is unfortunate it's not the end of the world, you just have to rebuild and move on. This problem is smallest for tech countries which tend to be very land thin.

At what point, if any, would it be appropriate for a farmer to start cashing instead of building cs's when they are in danger of becoming landfat?

Your assessment is correct except for Alliance server. On that special server, you get farmed if you don't have a tag. All tagged countries generally have some form of pacts with other tagged countries, and hence the only thing most clans can attack are untagged countries. So even if you are the most land thin country, you WILL get landgrabbed until you are in 9 DR (diminishing returns). To the exact second you exit DR even.

As a farmer, you basically should try to maintain over 200% agriculture tech at all points of the game (when stocking, at least 222+% non-Demo, 235+% if Demo). Anytime you start to fall in tech, grab 1-2 PS less that day, spend the turns cashing/buildcs and use that cash on tech. Res/bus/agri/weap techs. And a little bit on Strategy.