Highly variable. There is a wide range of mitochondrial myopathies, each with its own severity and expected lifespan. Some cause death in infancy, while others have no real impact on lifespan at all. A neurologist and/or geneticist can help you understand the specifics of your particular case.
Mitochondial myopath. Life expectancy depends on many factors. Some mit myopathies are severly progressive and some are mildly progressive. There are different variations in how mild or severe each persons myopathy is.