how long does it take to replace master cylinder

Depending on your level of experience, anywhere from 30 minutes too ???, It depends a lot on your knowledge level and available tools as well. You will need line wrenches ( also called flare but wrench) a decent socket wrench set and possibly a bleeder kit to remove air from the master cylinder when adding new brake fluid, You may also need to bleed your entire brake system and ABS unit , if so equipped as well, depending on whether you have assistance from a friend or mechanically knowledgeable helper, this can become difficult for some people. If youve never attempted this before, I would invest in a good repair manual, liked Chilton's or Haynes these are also available online for purchase .

Typo, flare wrench, not flare but wrench! I don't know where that popped in.