Effect of oxidation aluminium reflectivity
The lifetime of aluminum depends on the environment. If it's near an ocean, where air is humid and often contains salt particles, it may become pitted or look a milk white, rather than shiny, in just months. Anodizing can protect it for years, though. 

Because the surface of anodized aluminum is porous (until sealed), it may have a somewhat more or less matte finish depending on treatment. Bright dip anodizing, depending on the aluminum alloy, offers a more shiny finish.

The Alzak process is used for lighting reflectors, and there is information on its specular reflectivity.