That's like asking how long is a string? It's as long as it needs to be. For a declaration of factual innocence a great deal depends on what is in the file (including police reports), what you were charges with and why the case was dismissed. 1385 covers a lot of reasons the DA cannot go forward. If the case was dismissed because your abused girlfriend decided not to testify against you your not going to win your petition.
This will be particular to the facts of your case. You have to prove that the facts are such that there is no reason to believe you committed the crime. In some cases this would be very easy, in other cases it would be impossible.
The burden is on you, but you have nothing to lose so why not hire the best locally experienced criminal defense attorney you can afford who can assist and answer all your case specific questions?
Law Offices of David Shapiro 3555 4th Avenue San Diego, CA 92103 (619) 295-3555