I think I actually agree with 12 hours. I used to play Legends, which was quite a smaller shard compared to some of the larger ones, and we'd even have problems finding enough people to stay up a whole 24 hours. Things were a lot easier in the summer, but I know that I'm not going to be pulling all-nighters on most given days (being that I work, etc.) and I definitely wouldnt try that when school starts back up.
Guards, in my opinion, aren't too bad a problem, we go in with a few SL peacemakers and take them out when we can, but last night, we got a real nice sized raiding party going for once, and I remember saying, we're going to obliterate them, theres only three COM guarding, and in the end I got some funny screenshots of 20 guys in pink getting owned by 7 guards and two nightmares that were wandering the entrance after we had already taken out a good hanful of guards, at least 5 or so. If any changes are made, limit how many can be placed in a time period, that way if we actually wipe them out, they cant just keep replacing them right away. Magnicia especially has enough silver to keep that up for a good while, and it is hard to find good raiding parties nowadays (especially in all pink..)