Temperatures of at least 70 degrees keep the yeast happy so they eat the priming sugar quicker. For a normal gravity beer, that should only take a few weeks. Once they're carbed, you can store them a little cooler if you want, something closer to 50-60 (cellar temps) is best for long term storage.
Some darker beers tend to taste better after a few months of aging, but it's certainly not a requirement. It really depends on your recipe and process, it's very possible to have a delicious porter or stout in a couple weeks. But if it tastes a little harsh, sit on it for a few months and try another one.