If you go by online games: I have had 1 that started 1198 and went forward, one that was 1208 going forward, two for 1220 and one that I am looking entering with year off 1150's. So that puts 4 in the 1200's and 1 in 1100's
offline (vs onlie) I have one that was 1198 to about 1210 and my current one in 1060 (started 1056)
I think the key that 3rd edition was 1198 starting, 4th was 1208 (I think) and 5th is 1220 for base starting and the suppliments are all best for information on early 1200's play would make that most common.
Now, one member of my current offline saga would have to put it at around 1800 I think because while he is in our current 1060 saga and played in the prior 1098-1210 saga (when moving, and other things reduced group to 3, we shifted to D&D for 7 years before going back to ars in the current saga), he has run events at Gen Con for the last several years in 1800's england with a more modern order.
We have tended to often be vauge about exact year, allows the SG to bring in historical events more easily.
I think our earliest setting had late 800s as the starting point and the latest began in the 1300s.
Nearly all games have stretched for at least 2 centuries though and one didnt really end at all even after reaching 1600s(but 1 key player moved and the primary SG went "out of town" for a year so we "iced it" for the time being(and covenant including one soon to be 500+ years old maga hehe)).
I tend to start late 12th/early 13th century but it depends on if there are any historical events in the area I am going to set the game. I have plans for a game set in constaninople that will start about 1190 or earlier becasue of the crusade in 1204 .
I have started a saga in 1311 with my group, in the Normandy tribunal. I chose this starting date so that the players are in the front row to see (and play with, obviously) the end of the French Capetian dynasty (les Rois Maudits) and the beginning of the Hundred Years War and Breton War of Succession.
We started our last long running saga in 1060. It was set in the Stonehenge Tribunal and i wanted to bring in the Norman conquest of England and the loss of power of the saxons to the normans.
Since then, I've played in two games set in the 1100s - one in the 1170s when the Common Law is very new, one 1190s. Still, overall most sessions played is still in 1200-1300 period.
Most of the games I have played in start in the 12th century, even if they move far past that date. While the current one had an official start date of 1220, preplay (which took 3 months of real time) went back into the 1100s for a few of the characters. My character wasn't one of them, since his played through apprenticeship started in 1202.
Something from 3rd Edition (GASP The Demon crazy one?!) is actually useful to look at for time periods happening during/after the renaissance. That would be the "Realm of Reason", starting on page 332.
While it really did not make a whole lot of sense being in the 3rd core book, it does give some idea of what the renaissance and raise of rational thought would have had on the other Realms of Power. Think of it as mundane thought, as the masses move away from believing in magic and faeries (and to a lesser extent the Divine/Infernal). It will eventually become what White Wolf called Paradox in Mage: the Ascension.