Our longest running saga lasted 45 years, from 822 to 867. It was a fairly fast-paced saga. We played almost every week and sometimes got through multiple years in a single session.
Our current saga has lasted roughly 2 years, starting in 1098 and we are currently in spring 1100. It is a much slower paced saga, with multi-session adventures covering only a single season and only played once or twice a month.
Both sagas have proven to be great fun, in very different ways.
Of the three sagas I've played in, the first was fairly short, 5-6 years perhaps. The second was very long, covering over 175 years of game time (over two years real time and two editions of Ars Magica). The third lasted only about 35 years. My current saga is the first I've run as sole storyguide and we're only 11 years into the game (starting in 1196 and we've just entered 1207 last session).
Our saga started 10 years ago (IRL) and we've been through 51 years of game time. We're still on our founding magi, but they're walking cannon now so we have younger magi too. Our covenant has had different 8 players and one ST (with 2 of the players also running one offs and grog related adventured stories). We've had about 5/6 major story arcs over this time.
We've no intention to end as we're in Scotland in 1271. By the time we end the current arc (Damn you dirty beast). We'll be right into the time of William Wallace and all that jazz.
Sagas that just don't take off tend to run for about a decade at most. Some founder within 5 years, some even sooner.
The longest saga I've played in is one I'm playin in now. We started in 1220 and are currently in 1263, so 43 years. We play weekly, and tend to advance around two years game time for a month of real time. We have a real mix of magi - we have three of the founding members left, a 'later intake' of magi about fifteen years younger, and a second intake of magi about 30 years younger. Companions have come and gone: many have retired to a minor role within the story, some have died.
Our alpha SG has said he does not intend to let the game go beyond 1600...
As an aside, 'walking cannon' depends largely on the magus. We just ran an adventure where the group was two companions, five grogs and a magus whose combat prowess ranks similar to 'daffodil' despite being 20 years out of Gauntlet. It was a great fun adventure - going up against things that often the group would just blitz through and really struggling. And our compassionate, caring, loves-everyone necromancer learned that perhaps having a combat spell might not be such a bad idea.
IIRC, about four hundred years for the longest, but that was an extremely quick play running game. More for fun, practise running a covenant and testing house rules than any serious small scale roleplaying.
Longest more serious, somewhere in the 100-200 years range, i lost track of exact years since i was mostly playing opfor.
More normally 30-100 years. With some mixing of fastpaced and slowpaced this is a nice range to allow both longterm and action playing and still keep it to within decent realtime limits.
Mine have all been really short compared with most peoples. The ones that stick in my mind were Antiqua NAtura my first long running saga which only ran a couple of years real time and covered 1210-1226 and the current Monday night saga that has run from 1160-1211 (in about two and three quarter years real time i think?), with many stories lasting many sessions in some years, then a year or two downtime. So 51 years is my longest saga. However long does not necessarily mean best - our Mistridge Saga covering 1209-1217 covered the Albigensian Crusade and was one of my favourites, despite being less than sieightx months long and being played over two or three rounds of the Ars 5 playtesting process.
The slowest campaign in any system was over the edge, it was about real time if you calculate the session time versus the waking time of the characters. (just over a week in a three year biweekly campaign).
The current campaign had about half a year real time of meetings to compile the covenant and get to know one another before the charter was signed. We kept starting a session with the summary of last month's meeting, and ending with AOB's before closing the session for about half a year past that.
After a while thing sped up a lot, we are now some 25 years past the start, in about five years.
A good thing to remember when you expect a campaign to be shorter is to consider starting with characters a little older and more experienced, to avoid potential one-trick ponies ending up in situations they´re not ready for and doesn´t have the time to prepare for either.
I could not agree more with Direwolf75's suggestion.
Magi at gauntlet are nothing like magi at 30 years past apprenticeship - not just in terms of power, but in terms of goals and direction. As a SG, the shift in what magi do and how they operate can take a bit of a shift in play-style.
I'd recommend running a game with fresh gauntlet'd magi for starters, then if that game ends (either well or poorly), consider the next game being one where everyone is 30-40 years post-apprenticeship. Let them build up more powerful magi, get used to running stories for a summer covenant, etc. That way if the second game also collapses, you'll be able to start game #3 with a better idea of what you can grow towards, what older magi in other covenants are capable of, etc.
It also lets the players experience life as a magus who is wet behind the ears and clueless about the world.
The main current one has been going for 27 years.
The secondary is on hiatus but has been going for much longer as the concept was to accelerate until the magi are archmages, something like 60 so far, IIRC.