Having picked the time period already, I needed to select the scale of the game. Theater of War (ToW) has three to chose from; Strategic, Grand Tactical and Tactical. The strategic covers an estimated 4000 x 4000 sq. miles and uses the seasons, Spring, Summer, Fall and Winter for the campaign turns. This was too large for what I had in mind.Allocating a block of time in-game to resolve certain kinds of action wasn't at all uncommon in roleplaying games back in the day. Searching for rumors or a patron is a week-long activity in Traveller. Original, little brown book D&D uses the week as the basic scale of time-keeping, with one day devoted to dungeon exploration and the rest to rest and refitting: "The time for dungeon adventures considers only preparations and a typical, one day descent into the pits." A campaign turn in Boot Hill is a week or a month at the referee's discretion. The knights in Pendragon typically have one adventure per year. En Garde! turns are one week long, and the game also uses the month and the season as units of time in resolving action.
Grand Tactical covers a smaller area roughly 400 x 400 sq. miles and uses months as a campaign turn. So 12 turns equals a year. This is more in line with what I was thinking.
The tactical covers an area of 50 x 50 miles and the campaign turn is a week and the campaign ends after the 4th week or more.
The month also appears in Flashing Blades as a standard measure for resolving campaign-level action. Career commitments are measured in months, as is the time spent for gaining new skills, practicing martial skills, or pursuing minor jobs; frex, a priest's annual commitment to performing his duties is six months, with the other six months available for training, practice, pursuing another career or job, or, most importantly, adventuring. Interestingly, the time commitment for a career is flexible: "Each year, a Priest must spend six months, distributed as he likes, performing his religious duties." This gives many careers the feel of a sinecure rather than a nine-to-five job, but it's not inconsistent with how some offices were administered in l'ancien régime.
There is a significant exception to this flexibility: the military campaign. Soldiers and knights on campaign are committed for six months to action at 'the front,' which is resolved through a fun mini-game which condenses the action into three 'encounters.' These encounters begin with the risk of wounds, then may lead to a special event such as an individual battle with an enemy soldier while defending a strategic position, breaking out after being surrounded, or capturing an enemy's flag. Prisoners may be taken and ransom and booty claimed. Military campaigns are waged over six consecutive months, as determined by a die roll, and adventurers whose units are on campaign must serve. The campaign mini-game draws many FB players to military careers for their characters.
Campaign turns in wargames often condense large swathes of activity, particularly involving strategic movement and logistics. Avalon Hill's Kingmaker was a good example of this, with a player's turn involving moving his faction's nobles and their pretenders until a battle broke out or Parliament was convened, in which case the action changed to tactical level combat between forces or the negotiation and treachery of marshaling votes and declaring a king; The Rise and Fall of the Third Reich, another AH board-and-chit game, begins each turn with allocating basic resource points to simulate the logistics of production and supply, followed by strategic movement, and it's only after these are resolved that combat between fleets and armies may take place.
In a roleplaying game, campaign turns may be an opportunity for a mini-game, such as the aforementioned military campaign for Flashing Blades or the cattle drive mini-game published in Reilly Associates' Variant magazine back in the early Eighties for 2e Boot Hill, or they may be something akin to one or more sorts of montages. One could, for example, imagine a Flashing Blades' priest's six months of "religious duties" as a series of vignettes: collecting alms for the poor, praying in his cell, saying Mass, leading a saint's day procession, hearing confession, and so on.
As a referee, I often find myself asking the players something like, "Tell me what it is you want to accomplish," instead of asking for single, discrete steps - "You want to find out if Baron de Bauchery was in town, so first you'll talk to the innkeeper at the Black Swan, and after that you'll take to the innkeeper at the Roe Deer, then . . . " becomes, "So you want to make the rounds of the local inns to find out if the Baron de Bauchery was staying in town when Princess Pinkflower disappeared? Tell me how you plan to approach this." Do they offer bribes? Do they attempt to intimidate the innkeepers? Do they present themselves as allies of the baron? Smart players will use campaign turns similarly, as an opportunity to pursue long-term character development and engagement with the game-world; perhaps our FB priest will use his six months of pastoral obligation to become acquainted with a seigneur and his family, or to research conflicts between Catholics and heretics from the Wars of Religion in his parish.
Beyond a means of condensing routine, non-'adventuring' action, campaign turns also perform an important pacing function. Rising action isn't rising action with falling action for contrast, and campaign turns serve as an opportunity for the campaign to periodically catch its breath. This can help a game-world feel like a living place, where the adventurers follow seasonal rhythms or discharge their responsibilities as figures in in-game society. My first experience playing in a 3e D&D campaign found our characters climbing multiple levels in mere weeks in-game; coming directly from a 1e AD&D background, with steeper experience point requirements and rules such as required training intervals to gain levels, the effect was disconcerting. In my experience, allowing the campaign to proceed in verisimilar cycles contributes to immersion in the game-world.