In most states, the legal tinting limit on the passenger and driver's side windows is 35%. Most of the other windows in the vehicle can be tinted to 5%, but no darker, you still have to be able to see through the window if there IS a window in place.
On your windshield, most states will allow you to tint 8 inches down from the top, but you should check with your local laws to make sure of this.
<< I believe the key to avoiding the purplish/bubbled mess is to get metallic film tint... I dunno about brands or anything, but I did pick that up from a recent thread. >>
It doesn't necessarily have to be metallic film, but companies do have different guarantees on tint.
When you go to the shop, make sure they have a demo vehicle or two on display (usually the owner's car or the tinter's car) and ask to see the tint job up close. Also, ask the shop the specific warranty on the film. If you get a film which has a lifetime warranty (most shops which ONLY do tinting, or only tinting and a couple other things, will offer this as a choice) you should be fine from avoiding purplish fades. Avoiding the bubbles is all a matter of the tinter's individual abilities as well as you not rolling down your windows for approximately 7 days.
dm