As nasastar said, most states have tinting regulations. Please ask tinting shops about what the visibility requirement is in your state. Generally speaking, all windows behind the driver seats have no requirement, i.e. you can have any film ratings like 20% even 95%. But two front windows should have 35% or higher.
Nevertheless, tinting law is a secondary one, i.e., a cop can't stop you just because s/he sees your windows being too dark, s/he must have other legitimate reasons to stop you for primary violations, then may give you a citation for the window non-compliance. In the matter of fact, window laws have not really been enforced in any states.