There really aren't hard-and-fast rules for aging. And you're not likely to go through a keg in less than a week, so it's aging as you drink it...if it's really a hell of a lot better by the time you get to the last pint, age it longer next time before you tap it.
If you're letting a lot of beers age in a keg for a substantial length of time, you have more patience and storage space than most
A good starting point for total cold-conditioning would be this:
Basic low-gravity (< 1.050) ale - 2 weeks
Mid-gravity (<1.060) Pale Ale, etc - 3 weeks
IPA, Porter, etc - 4-weeks
Basic Lager, DIPA, Stout etc - 4 weeks
Complex Lager 4-5 weeks
Higher-gravity Ales, lagers, etc - 6 weeks +
Barleywines, big Scottish Ales, etc - many months
Others may advise a little differently, but you wouldn't go too far wrong with that.