For most stouts, porters, and browns, a month is pretty good time. If it lasts longer, and you like it better that way, let them age longer.
Foreign extra stouts and, certainly, imperial stouts, need more time. Many imperial stouts need six months to a year or more before they peak. Baltic porters are the same way. Hoppy American browns should be good after a month, but they may improve with more time. A particularly malty brown also might like some more time.
Wheat beers, other than weizenbocks, are usually designed to drink young. Weizenbocks can take months.
And, I guess when we talk about "beers" here, we're talking about ales. Lagers need some good lagering time. As you see, there is no rule or even guideline for aging "beer" anymore than the term "beer" describes every style.