I found this link posted in another thread

http://tintlaws.com/

From the site:
HOW DARK CAN WINDOW TINT BE IN MARYLAND?
Darkness of tint is measured by Visible Light Transmission percentage (VLT%). In Maryland, this percentage refers to percentage of visible light allowed in through the combination of film and the window.

• Windshield Non-reflective tint is allowed on the top 5 inches of the windshield.
• Front Side Windows Must allow more than 35% of light in.
• Back Side Windows Must allow more than 35% of light in.
• Rear Window Must allow more than 35% of light in.

HOW REFLECTIVE CAN THE TINT BE IN MARYLAND?
Similar to sunglass lenses, some tinting film contain metallic elements that help in reflecting incoming light and reducing the glare and heat generated by visible light.

• Front Side Windows No reference to reflectivity in the current law.
• Back Side Windows No reference to reflectivity in the current law.

OTHER MARYLAND RULES AND REGULATIONS

• Restricted Colors The tint color(s) of RED are not legal by state law.
• Side Mirrors Dual side mirrors are required if back window is tinted.
• Certificate Requirements Manufacturers of film need to certify the film they sell in the state. Ask your dealer if they are using ceritified film.
• Sticker Requirements The sticker to identify legal tinting is required between the film & glass on each tinted window.
• Medical Exemption State allows medical exemptions for special tint. For more details about the specific terms of the exemption, consult your state law.


SO the answer is, it depends on how dark you go.