All people who have access to the server can claim some land to build on.
As space in the FTBCity is limited and almost full, we would appreciate that players who don't intend to really use their city plots or don't intend to play much, don't claim one, so that the remaining ones are free for other players to use.... but that's really the only claim-type restriction.
We have no formal defining methods for claims. A signpost letting people know that not only the area is claimed, by by whom - is a very good idea. As is marking out the boundaries (or corners) by torches and/or blocks, so other players can get an idea of how much land you are claiming (I tend to claim more than I think I'll need, since you always end up needing more
Some people like to have the land to themselves and not have a neighbour within view - so it is courtesy to make sure your claim is not within view distance (on maximum"far" view distance) of someone else, unless you've checked with that person that they would appreciate a close neighbour.
Likewise starting a mine, treefarm or other construction you may not consider your "claim", within view distance of someone (without their ok) is impolite for the same reason - some people choose their locations because of the terrain around them, and changes to that terrain may not be well received.
For example my house is in a valley, so you could say my official "claim" is only within the walls/fences I've placed around my house - but I would be annoyed if someone chopped down the forest or mined away the mountains surrounding my valley - as it's all part of the reason I chose that particular valley to build my house in.
What is an acceptable grace period when someone has jumped in and contact is initiated prior to removing the blocks? One week? Two? I would gather this latter part would be something up to the admins to handle/authorize.
So are you meaning how long after a player seems to have abandoned their claim, can someone else claim it? Never. A player's claim/building is always left as is, even if they have left the server and are not intending to return (with the exception of FTBCity plots). There is plenty of space, so their creations can remain untouched as part of TOGMinecraft history.
Also, people can be actively building underground, so the surface may not change, but underground may.
Or do you mean how long to leave it if you ask someone if you can build within view of them? Personally I'd say you should not build if you've not got their ok, as again, there is plenty of space and some people don't like neighbours in view.... but in the past, we've tended to have the policy whereby people post a pic on the forum of their proposed claim (from the world map image), and leave it a week for people to give any objections.
With the new mapping it is now not possible to see who has settled in a location (unlike tectonicus which showed players and beds).... so that makes it harder to see who has a particular claim... which is where signposting whose place it is, is important.