A good tint/stereo shop should be able to help you with the following:
They should know the local law and be able to inform you about it.
They should also have samples of different darkness of tint, colors, etc. on glass that you can take out to put on your windows to see what you like.
Some may warranty it for life, or a couple years.
I highly suggest against "limo tint" usually around 5%, meaning only 5% of light coming in. The police do not like it because it is so dark they can't see what is going on inside. As a courtesy, I will even roll my windows down so the police can see inside even if my tint is legal. It shows you have nothing to hide.
In WI I run 20-17% on everything but the windshield and the rear window, which really makes the vehicle look nice. Law is 50% front 2, 35% rears, windshield is illegal. Your laws may differ. If you get caught running darker, you will have so many days to take it off and show the police. I have had Troopers pull out a light meter on me to check my windows when I used to run them pretty dark.
Mine isn't too dark but still noticeable, even though I am still breaking the law. The reason I don't do the rear window is because the darker the tint the harder it is to see at night out the back when backing up.