Many factors determine the length of state post-conviction proceedings, including the jurisdiction, judge, and issues in a case.
On average, state post-conviction proceedings last between 4 - 7 years. This appeal is the best and sometimes only opportunity to create a record of the errors that may have occurred at your client's trial.
Federal habeas proceedings follow the completion of state post-conviction proceedings. Under the current law, additional or "successive" petitions for relief are strictly limited to extraordinary circumstances.