There is a legal limit. I have just that.
The legal limit for tint is 35% on the front windows (Driver and passenger)
And 5% for the rear of the car. (Back window and Back side windows)
I Know this because when I got my tints They wouldn't let me go any darker than this. The tint shop (Xtreme) told me directly that this is the legal limit and that he wouldn't do darker for me because he could get fined or something.
The legal limit is crazy dark in the back. You cannot see in my car through any of the back windows. The sides don't even feel dark at all. People think it makes it hard to see out the back window But truthfully it's not that bad. I manage just fine.
Here's pics of my tints when I first got them done!
http://forums.vitalmotion.net/showthread.php?t=27142