First of all, let's look at the definition of SERVICE DOG as defined by the ADA...
What type of training should a dog go through before becoming a service dog?
A service dog is a dog that is individually trained to perform work or tasks to help mitigate a disabled person's disability.
The answer to this question does vary. It depends on what kind of disabilities the dog is being trained to assist with and whether the service dog will be only used at home or if it will also have public access. There is no uniform standard training for service dogs as each will be assisting with a variety of disabilities, and no one person experiences the disability the same as another. The "Cookie Cutter" service dog training school does not work for all disabilities. That being said, there are untold standards that must be met.
How long can it take to train a service dog?
How can you tell a service dog is ready to work as a service dog?
Is the dog potty trained well?