I don't think you'll find a published absolute maximum length. Since there's a cat5e and coax cables running between the ONT and BBU, it's safe to say the length of the cables before losing signal strength might be the determining factor. I suggest you ask the technician when they show up before the install.
I asked the same question because I wanted the BBU to be in a house closet near an electrical outlet. From the ONT up to the actic, down the wall into the closet is about 50 feet (about 40 feet as the crow flies). No problem.
Written by FiosFAQ - Ron
Last updated on 12.20.06