What is the maximum spacer thickness I can have on the fronts and rears in my car?

Or if Max is difficult to define...what does everyone else run?

cheers

 

i run 18mm MDF spacers in the front for my Focal K2P components.The door card just about sits back in its original position without touching the speaker,i think thats about as thick as you can go without having custom pods made up.I havent bothered with the back as i don't listen to music there