Master cylinder - not that often at all. Sometimes never for the life of the vehicle. Was there a description of what is wrong with it?
Brakes - anywhere between 15,000 - 50,000 miles per axle on any given vehicle could be normal. Depends way too much on driving - city verses highway, etc etc.
Struts - for wear and tear - probably would start looking around 60,000 miles. I'm getting that the left front strut may be actually leaking, which is necessitating replacement. Again, get a good description of why it needs replaced.