How long did Maniac live with his aunt and uncle?

The answer to this question can be found in chapter 1 of Maniac Magee. When the chapter begins, readers are told that Maniac was born in a great house with loving parents. Unfortunately, his parents were killed in a tragic trolley crash when he was three years old. Maniac was then "shipped off" to live with his nearest relatives. Those relatives are Aunt Dot and Uncle Dan. Their marriage is terrible, but they refuse to get a divorce because they are Catholic. So instead of getting divorced, they just live completely separate lives in the same house. They have two of everything so they don't have to share with each other. The text tells us that Maniac grew tired and frustrated with this arrangement, and after 8 years of it, he finally had enough. Maniac ran away.

Eight years of that. . . . And that's when the running started. Three springy steps down from the risers—girls in pastel dresses screaming, the music director lunging—a leap from the stage, out the side door and into the starry, sweet, onion-grass-smelling night. Never again to return to the house of two toasters. Never again to return to school.