How does 'once or session' work in PBP?

There are several game systems that has a once per session mechanic of one sort or another.
Since we don't meet and have a game session, how do we embrace this mechanic?

Once or calendar month of real time?
Or is it something the GM should keep track of?

Love to hear your thoughts on the matter

I don't know of any 5e mechanics, but some books make mention of sessions.
I think it's by feel, by what "feels" like a session.

An IRL session probably has two or three encounters, and there'll (hopefully) be some sort of cohesion to a session. In PbP it's probably linked to the number of posts.

Dunno. It's a good question.

Always a tricky one. I vary my strategy based on what the game does with these sessions, sometimes that whole mechanism can be dropped with no loss.

It is sometimes possible to change the terminology from 'session' to 'chapter' (though these days people seem to be more inclined towards calling them 'episodes'). It is often obvious where these divisions happen.

The challenge is when there are resources that are available for a session and renewed for the next one (Fate Points, and the like). These can be lost if not used up. At the table the players can have some sense of when a session is coming to a close (they all know that Bob needs to be home before midnight and it is 22h45 already) so they can hurry to use them (not that I like that, but it is part of the rules). The GM may need to telegraph that the 'chapter' is drawing to a close so the players can know to stop hording and spend.

There are systems what have you check if certain events have happened during a session and award XP, or something. Often the easiest is to note the first time each happens (or the first time you notice) and then reset that count when the time seems right (maybe after everyone has had a chance at most of them? maybe for every 'scene' (the Singleton Rule)?). This allows the impression of quicker (or at least 'sooner') awards, while also allowing some control over how fast they come.

If you have specific questions about specific games or rules, we may be able to provide more concrete examples.

It could work to end a pbp session on an arbitrary number like time or number of posts. It's straightforward and predictable and fair.

The main drawback I see is that it isn't tied to the story at all. IRL sessions have a logical beginning and end. If you count a session ending at the end of the month or after X posts then you could end a session in the middle of a conversation or at a generally slow moment in the story. That feels wrong.

I suppose it is also possible for a bit of player metagaming with arbitrary session limits. Like, if you know your session expires after 100 posts and you're at 92 posts, you might conspire with your fellow players to pump out 8 low-quality posts just to get a fresh session. For monthly sessions, if it's May 30th you might stall your post for 2 days in order to get that fresh new session.

I think the most natural way is for the GM to just pick a good moment to end a session and start a new one. Choose a moment that feels appropriate in the story, like after a hard fight or achieving a short term goal. It's okay if you're not consistent with duration; IRL sessions vary in length too.

Similar to others who have commented, I've been doing this by chapter or episode. Deciding were the cut-off lands, I've just done that by feel as dictates by the narrative.

Personally, I play a lot of Fate, and I usually have characters Refresh their Fate Points after two or three encounters. I don't always know when I'll stop one thread and move to the next, so I use that benchmark instead.

I think number of encounters is the way to go. If you've never or rarely played live then this is hard to judge. But three encounters of a type and difficulty that players might use per session resources is probably a good starting point.

While that might seem quick, in PbP those two or three encounters can take a long time, and if such mechanics are exercised regularly people tend to forget about them.

Keeping it flexible --three quick encounters, or two regular length ones, or whatever-- can cut down on the meta-gaming.

It's kind of a judgement call. I know in the past I've re-created gaming intervals around things such as once per "rest" because that could fit comfortably into how time is organized in the games, rather than once per session.

I usually use something along the line of "every 3-6 scenes/encounters" depending on their complexity or importance.

This is something I’ve been thinking about in my Cthulhu games. Historically I’ve been giving rewards at the end of a scenario but actually a normal session might not span the whole scenario if played over the table, so I’m reconsidering my approach.

For my Savage Worlds game, I divide it up into chapters when it makes sense to end one (usually at the end of a minor campaign arc, which works out to every couple of scenes). Each chapter is a "session."

I would say either number of combat encounters (probably using 3 combat encounters) or number of distinct geographical scenes (in which case I'd use 5 distinct scenes probably). But it all depends on your game and the pacing you prefer :-)