Exactly, its anecdotal and subjective. That makes it very, very difficult. It'd be more realistic to try to prove titers/protection from a vaccination than prove some of the "diseases" that people are contributing to the vaccine. If protection was proved for a longer period of time, it justifies less frequent vaccinations. that might open the door for investigating soem of the "attributed diseases". But the AVMA and others (and so does the CDC for human vaccines) acknowledge immune problems, digestive, etc with a vaccine related cause. That was the whole reason for the change to the 3 year protocol, pets were getting more and more problems.
I really don't have a set time to answer your second question.
Like I said, if I have a healthy dog and he gets his shots and 2 months later he has a sudden chronic problem, or I had a happy, well adjusted pet that suddenly becomes aggressive after his vacs, to ignore the connection would be asinine. And I have heard of MANY people who had a dog that got along great with other dogs suddenly want to kill any they saw after a vaccination, or becomes stranger fearful. It's the same bullshit that is told to parents of autistic kids who had a happy, healthy child, it was vaccinated and now is a different person.
It's double speak, "yes vaccines can cause these problems sometimes, but you have to prove it" so in reality none of these cases exist because no one can prove it! If it doesn't happen immediately after the shot, their can be no connection. That really is pretty sad.