I generally age the beer on the yeast cake 7 to 21 days after fermentation is complete, at fermentation temperatures. (Typically I ferment in the 65-67F range.) The flavor of all beers change during this time, but it is very noticeable with Porters and Stouts.
Because there are so many variables, it's really impossible to give an exact aging time. My suggestion is to taste it, and actually observe the changes in flavor. (I really didn't care for the flavor of the first porter I brewed, but after three weeks it was very very nice.)
Note though, clarifying agents like gelatin work by bonding with haze forming proteins which make them settle more quickly to the bottom of the keg / fermentor / bottle. If you are going to keep this keg cold (as in a degree or two above freezing) until December, you probably can skip the gelatin because with that length of time even the cloudiest of beers should clear on their own. (Just don't shake the keg!)