Effect of oxidation aluminium reflectivity Aluminium is a fairly good reflector, but oxidation will reduce it's optical reflective properties.

Aluminium is used for reflectors either being deposited on a substrate, or polished and protected by a protective layer to avoid oxidation.

How will the oxide layer of a several year exposed polished aluminium (indoor) will impact the reflectivity.  

Will it be considerable or minimal ?
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.