The long term campaign is the ideal gaming situation for many people (me included).
There's really a couple of different types of these.
There's the long term "module" type campaign where there's no continuity between sessions, but there's continuity between characters. I guess Living Greyhawk is like that..(if taken at a larger scale- I tend to think of the Living Campaigns as their own thing) but I've seen a couple of regular campaigns that work like that as well. Players get together for one adventure or 'episode', but it usually doesn't have all that much to do with what happened last session, unless it's specifically a continuation of that module or adventure or whatever.
The kind of campaign I'm really talking about is the one where the game session pretty much starts where the last session left off. Perhaps a day went by.. or even a week or more. But the continuity is ideally unbroken.
The interesting thing about this second type is that there's ususally no 'modules', or if they are, attempts are made to phase them in seamlessly.
I firmly believe that this second type of campaign is the most common type of campaign. Mainly, I base this reasoning on the idea that adventures take a long time to write if your'e trying to create self-contained episodes. But in this second way, you just have to create individual encounters, individual NPCs, and individual entities like towns and whatnot. If your'e a bit lazy, (I am) you can even run encounters straight out of the monster book.. There's a lot of flexibility and improvisation going on.
Anyhow, with the first type of long term campaign, you don't need much frequency. If all you do is play at conventions or in modules, there's no urgent need to keep track of what happened last session. You can also pretty much handwave or even invent anything that was supposed to happen between sessions.
With the second type, frequency is kind of important. I tend to think minimum frequency for a continuitous campaign is weekly. See, continuity is very cool when you are developing your individual characters story, as well as the shared story of your campaign. The more details that come together, the more interesting and satisfying the story becomes.
Anything less frequent than that, and you start to lose more than you gain from the benefits of continuity. As each player starts to forget little details, the campaign gets pulled in different directions, and it's quite hard to make the campaign "go" anywhere. I've often seen it as someone asking 3 months down the line "wait, what are we supposed to be doing again? I can't remember how this mission got started..."
Thats actually a frequency issue.