It takes 2 years. And your appeal will be denied.  You'll be given one more chance to appeal to someone higher up, and that appeal will also be denied.  At that point the letter they send you will say something like "this is our final correspondence with you".  When I did it, I chose not to waste money on a lawyer and an in-person appearance.  I just wrote an appeal letter and moved on.  I was surprised to get the certified letter two years later from the good ol' agency.

Care to share your story?  Why was your clearance denied?  Did you polygraph and do your 3-day processing at the CIA Dulles Discovery Building in Chantilly VA?