The Wall Street Journal has an interesting article about the length of time that recent Supreme Court cases from the last five years were pending by the time they reached the apex court. This article only considers cases where the Supreme Court granted a writ of certiorari which is a very small percentage of the cases that are appealed there.
Not surprisingly most the cases were pending for 4-6 years. Criminal appeals and some interlocutory appeals take priority, but after conviction criminal cases can take far longer than civil cases where collateral habeas corpus review is sought especially in death penatly cases. Some cases may also be remanded back to the lower courts and come back for further review. Other cases may first reach the appellate courts on preliminary procedural matters and once those are resolved return on appeals from the judgment.
http://online.wsj.com/articles/why-supreme-court-cases-are-marathons-1417394399
The Law Office of Kurt T. Koehler, 308 1/2 S. State Street Ann Arbor, Michigan (MI) 48198 (Washtenaw County); Copyright 2012 by Kurt Koehler