What lazycis said is very true. “[w]hat constitutes an unreasonable delay in the context of immigration applications depends to a great extent on the facts of a particular case.” Yu v. Brown, 36 F. Supp. 2d 922, 935 (D.N.M. 1999).
The courts have a discretion to determine whether the delay is unreasonable or not. It depends on the facts in each individual case, that's why it's impossible to say "All cases more than 6 month old are unreasonably delayed" or "There is no chance to win a lawsuit unless your app is pending for more than 12 months". In some cases, for example, an applicant was slow responding to RFE or provided incomplete response and an additional RFE was necessary. Court cannot force agency to act quickly in such situation.