Generally, the higher the OG, the longer you want to let the beer age. However, I have found that almost all brews ( with the exception of a few such as wheat beers) benefit greatly from 4-6 months aging from brewday to drinking, however, you wouldn't want to let it sit on the yeast that long. Some of this time can be spent in the keg. Many will tell you that you don't need to secondary, and I find this to be true, though I still do it when I want to age a beer before kegging. You can easily keg several weeks to a month after brewing, and let the beer condition in the keg as well. I usually do whatever time allows.