Well, first of all, it's not essential to have an accurate computing. It's just a forecast based on averages, the exact real duration is impossible to guess. So this is not the point.
That said, each year has 5 phases (Spr Dip, Spr Ret, Aut Dip, Aut Ret, Aut Bui). Yes, sometimes they will be only 4 because nobody will have to retreat, but it's rather rare. Further, sometime the very first turn could be repeated because of a CD. So, with a good approximation, a little rounded up, we can assume it's 5 phases/year. That is to say that it's 5 phases every 2 turns. So, if our stat says AvgDuration=15 turns, we could say:
Phases = AvgDuration*5/2
Phases = 15*5/2 = 37.5
Now, looking at the game pace (hours/phase), it easy to turn phases into hours and then hours into days.
Players would be warned that this is just an estimate.
The Point is that, since many CDs depend on Real Life issues, this way beginners, unexpert players and compulsive joiners would all have a Real Time forecast on how long their committment will be.
I've met beginners who told me "Hey, will this ever end? I thought it would have lasted 10-days or a couple of weeks...". Instead it was a couple of months...
So the point is not if 60 days is a forecast more or less correct than 70 days. The point is that:
- Many players (beginners, mainly) would discover that a game lasts longer than they believed and perhaps they won't join it if they realize they can't finish it (practical purposes).
- Any user would know that this Community expects that they _commit_ playing games until the end without NMRs (teaching purposes).