In The Watsons Go to Birmingham, for how long has Grandpa Sands been dead?

Byron's behavior has gotten so bad that his parents, Wilona and Daniel, can't figure out what to do with him. In desperation, they arrange a trip to Birmingham, Alabama where they will visit Grandma Sands and have her try to discipline Byron. They believe that Birmingham is "the real world" and Byron is sheltered in Flint, Michigan and they want to expose him to the real world in hopes that it will make him realize that he needs to shape up or he will have a tough future ahead of him.

The Watsons make the trip down to Birmingham, Alabama to visit Grandma Sands and leave Byron with her for the summer. When they arrive, Wilona is upset to learn that her mother has a boyfriend, Mr. Robert.

To comfort Wilona, Grandma Sands says "Your Daddy's been gone for almost twenty years."

We can tell that Grandma Sands and Mr. Robert have been together for almost that long when she says

We been good friends since right after you-all left for Flint--

and

Good, good, see, there's lots of things you can do down here, Mr. Robert ain't as much help as he used to be, so all them things he used to do you can do now.