Quick Answer: 7 days.
Ghost guilds have two other possible uses not mentioned.
They can serve as a highly mobile defending force for a guild which holds a lot of territory, and are often employed as a border guard, particularly if the main guild has sectors near an invasion zone. Other ghost guilds, or guilds not currently holding territory on a given map, if unimpeded, and having the required amount of goods, can take an invasion zone hex (the beaches), then siege and take the next hex inland, move their HQ there, and then siege and take all hexes which surround their HQ hex. Taking those hexes back immediately may be cost prohibitive for the main guild, but it's ghost can neutralize the threat, if it even let the threat get past in the first place. They can also just stay off a given map, and jump into the fray if a serious incursion takes place. The best defense against a ghost guild is another ghost guild, basically.
The other use is for laying point sieges, which both the ghost and the main guild benefit from. If you see a huge swath of territory belonging to one guild, with another guild holding two hexes in the center of that territory, you have likely spotted a point farming operation. The ghost in the center will release one of it's hexes, lay a siege army on the free hex consisting of a mixture of champions and rogues, take the hex to 1 army, and wait until the main guild has taken it's siege army to one or two armies before retaking the hex. As every member of a guild can release up to 4 hexes per day, it's quite easy to rack up the PvP tower points in this fashion.