How many years was Boo Radley locked inside his house in To Kill a Mockingbird?

Boo Radley did not leave his house for fifteen years.

The town of Maycomb holds many secrets, and one of them is Boo Radley. He was famous for once having stabbed his father in the leg with scissors. 

When Boo was a child, he got into some trouble.  He was called Arthur then. He ran with a gang of trouble-making juvenile delinquents.  Boo and his trouble-making friends finally went too far when they resisted arrest and...

Boo Radley did not leave his house for fifteen years.

The town of Maycomb holds many secrets, and one of them is Boo Radley. He was famous for once having stabbed his father in the leg with scissors. 

When Boo was a child, he got into some trouble.  He was called Arthur then. He ran with a gang of trouble-making juvenile delinquents.  Boo and his trouble-making friends finally went too far when they resisted arrest and locked the town beadle in an outhouse.

The judge decided to send the boys to the state industrial school, where boys were sometimes sent for no other reason than to provide them with food and decent shelter: it was no prison and it was no disgrace. Mr. Radley thought it was. (Ch. 1)

Instead of going to jail, he got a sort of house arrest.  However long that was supposed to have lasted, Boo became a recluse as an adult and simply never left his house.

The doors of the Radley house were closed on weekdays as well as Sundays, and Mr. Radley’s boy was not seen again for fifteen years. (Ch. 1)

It was after this that the incident with the scissors supposedly took place.  Boo stabbed Mr. Radley, and his mother went for help, screaming that he was killing them all.  When the sheriff arrived, thirty-three year old Boo was sitting on the floor cutting up a newspaper.  His father refused to send him to an asylum, so they locked him in the church basement.

Eventually, Boo returned home and became the town recluse.  His story was so well-known and sensationalized that he became the local boogeyman, blamed for being a peeping tom and accused by children of all kinds of scary deeds.  In reality, Boo was just quiet and shy.  He didn’t leave his house because he had no friends and was afraid to.