Baltic Porter Secondary Fermenter Time

I recently brewed a Baltic Porter which should finish out at about 7.9% ABV.  After two weeks in the primary, I transferred it to a secondary fermenter.  I have read in a few different places that the secondary fermenter time for higher alcohol beers should be around two months.  I was wondering why such a long period of time?  Does the beer mellow somewhat over that time period?  For all my other beers (around 5% ABV), I usually give them about 2 to 3 weeks in the secondary just for clarification.  This is a very dark beer and besides some yeast dropping out of suspension during this period, what else happens in the secondary over those two months?  I am guessing it has something to do with the high alcohol content?

Yeast continues to ferment.  Take gravity readings and let that tell you when its done.  At hi alc, the yeast works slow and hard.