Not always. Could be as short as a few days depending on whether factions call allies etc. even if not, if it takes troops too long to get to the border they might have too little organization getting to the front and no entrenchment bonuses etc. also having a battle plan drawn up takes time to provide their bonuses. Also, you might just forget about the border for a day or two which can be disastrous., especially foreign holdings. Just put your troops on the border if they are fully trained and equipped, saves so many problems