Something came up in our private forum.
We play in the Greater Alps tribunal (I just love this book), the tradition requires the wiazerd's war declaration to be brought to the victim by a Merce and 2 Red Caps.
How frequent wizard's wars are in your different campaigns ? Our local mercere is already annoyed at this 
François
Not much. A WW is a "duel at dawn" type of event. You can get out from it the victor, but if the enemy is also resourceful you can end up dead even if you are a good shooter. People only relies to outright violence like that in really dire circumstances and where other methods of solving a conflict (including the "if you do not move 3 tribunals outta here we are gonna rape you so hard your grandchildren are gonna feel it" statements) have been tried and failed.
Of course there are all tjhose trigger happy mages out there, but in general they are living glorious and short lives facing dragons and the like.
When theya re declared, WW tend to be reason enough to make one of the previous demands fulfilled and this is where the conflict ends. It is either that or mass-WW declaration, counterdeclaration and death of a whole covenant in less than a 6 months period. In those cases the merceres come along with a quaesitor that sanctions a longer WW period as long as certain condition is fulfilled without the need to have merceres hiking between covenants.
IMS 2 WW have been declared by my players. One ended with the destruction of the covenant and Toledo in one of the sillyest sagas ever( but man!, was it GLORIOUS!! 
). The other ended with the PC dead and a slightly pissed off player that saw cunning defeat brute force. He learned from the mistake and followed the plain hotheaded 3rd edition vanilla flambeau with a Bjornaer Mentem bastard that became one of the most feared mages in our tribunal in a fairly long running saga.
For a general-wide tribunal, I would say that 1 war per year would be fairly normal. Those wars will last slightly longer than a Moon, usually, so 2-3 declarations per year seem like normal stuff.
Cheers,
Xavi
I think that you should really consider the larger political ramifications of Wizard's War when you calculate how often they occur. The history of dueling would suggest that they might be fairly common among young hotheads, but I think that magi, as individually powerful and connected individuals, might be more prone to proxy fights and would bunch together in families, covenants, and groups, protecting and exerting control over their sodales. I would assume that there would be significant political ramifications to Wizard's War at Tribunal, and that it would have to be a fairly rare occurrence to have a full-out Wizard's War. That does not mean that there aren't powerful implications to the threat or declaration of Wizard's War or that unpopular magi wouldn't be particularly vulnerable (whereas a weak and popular magus might be considerably protected politically and by the threat of a broadening war).
Think about what the magus' group or faction gains from the Wizard's War, and whether they would support or censure him. Is he exposing them to attack, political or otherwise? Can they afford to lose him if he is defeated? Make these complex and restricted, but then remember that a lot of people are jerks and hotheads, and can likely not be dissuaded in any case--what do factions do then?
Interesting timing on your question since my magus just 2 weeks ago declared WW...the first war we've had in 13 or 14 yrs. of playing this game! I agree with everyone's input so far, more or less; I hardly think they're a weekly or monthly occurrence. At a guess I'd say it varies widely from 0-3 per year per Tribunal. And they probably come in clumps, like you might have a nice stretch of 2, 3 even 5 or 6 years w/out one, then one happens, then the dead wizard's faction/allies exact retribution, then maybe there's a counter-retribution. Probably most of the time these brushfires burn themselves out as magi either lose interest or die too much, but occasionally a particularly virulent animosity will have to be addressed at the next Tribunal. S**t happens, y'know?
As it happens, my "war" resulted in the other magus packing up and moving out before the onset of hostilities. Anticlimactic, I know, but he and I both knew he was outclassed and outgunned, which may be a fairly common precipitating factor (or not).
Depends on where you are. Our saga is set in the Rhine, and we think that wizard wars are rare in such a settled Tribunal. OTOH, they are much more common in the wilder Tribunals (e.g. Stonehenge, Novgorod and Loch Lleglan). We'd also gone for a romantic interpretation of Provence, with magi declaring (though not necessarily seriously fighting) wizard war all the time there as a matter of honour. Which is a recipe for a culture clash...