Miniature Schnauzers have a lifespan of between 12 to 15 years although I have a coworker at my office job that has a Miniature Schnauzer that is 20 years old but that's rare.
As long as you keep the Miniature Schnauzer healthy and keep up with Vet Visits if possible the Miniature Schnauzer should have a long healthy life of at least 15 years.
Be sure to give the Miniature Schnauzer at least 30 minute walks everyday to give the dog enough exercise which is healthy for the dog as well as healthy for yourself.