Most RPGs address the idea of breaking your game into discrete blocks, typically one or more campaigns, each of which is composed of several adventures.
Some groups play multiple campaigns with the same PCs, with each campaign having a distinct endpoint. Others play one long campaign, with smaller endpoints throughout. And there are games that have definite endpoints built right in (many indie RPGs do this).
There’s a sweet spot in there somewhere — the Goldilocks spot, if you will, where each story arc or campaign is just the right length for your group. So how do you find it?
Although this is a tricky question (and one without a single, definitive answer), there are several ways to make sure you’re on the right track.
Make it part of your social contract. If you have a social contract discussion with your group before starting the game, this is a great issue to bring up. Are they interested in one long campaign? Several smaller ones? Story arcs that span a few sessions?
Set realistic expectations. Does your group tend to abandon games after a few months? Are they in it for the long haul? Whatever the case may be, plan accordingly. (Just make sure to leave enough wiggle room for them to change their minds, and stick with it for more or less time than you expected.)
Adjust your approach during play. Have expectations is all well and good, but they might not survive contact with reality (and your players). Son’t be afraid to change the formula a few sessions in — just be sure to consult your players before making the shift.
Get feedback. Getting feedback from your players is always a good idea, and this is no exception. If you’re not sure the current setup (whatever it is) works for them, just ask.
Keep tabs on the game’s momentum. If you use a Loved, Blah, Hated List when you play, watch out for multi-session doldrums and other symptoms of large-scale pacing problems. Not all players enjoy giving feedback, and this is one good way to take their pulse about the structure of the game.
Where’s your group’s Goldilocks spot? And what have you done to find it — or are you still looking?