1.5 mm of pad material is quite adequate.
Most people don't understand how a hydraulic brake works. The piston seals are thick rubber and almost square in section with the pistons fitted very tightly indeed inside them. Try to pull a piston out and you'll appreciate how damn tight they are. The pads are pushed onto the rotors as the seals deform into a parallelogram shape then when you remove the hydraulic pressure they revert to their un-braked position as the seals resume their normal shape. The brake is a simple hydraulic pump with a reservoir of fluid that takes up brake wear. As the pads wear the pistons do creep slowly through the seals but the movement is microscopic. If for some reason the pistons aren't creeping through the seals you can begin to find increasing clearance and hence increasing lever travel as the pads wear. Some people mistake this increased movement for air in the system and they suggest pulling the levers to the bar for a few hours with a rubber band, thinking this somehow dissolves the air in the fluid or something equally daft but my guess is that actually they are just forcing the pistons to creep through the pads a little.
Sometimes if you grab a big handful of brake you can force the piston through the seals and you end up with a brake that's stuck on, which means you've got to remove the wheel and push the pads and pistons back in.
