said by fttnmiles :How far does my docsis 3 modem have to transmit
With modern DOCSIS 3.1 and DPoE standards, it's accurate to quote "as far as where fiber reaches near your home".
At your doorstep, on the poles outside your house or at the end of the street where you live.

There's DPoE(DOCSIS Provisioning over EPON) media convertors where minimal changes need to be made to your current home cable install.
The only changes if the drop fiber to your home, a DPoE/RFoG media converter in between your existing home coaxial network will do.
This is the Cable MSOs secret ace card beyond DOCSIS 3.1. When one day all your telephone wiring/cable TV lines on the utility poles are replaced 100% by fiber, they can still sell their cable services simultaneously along with IP based PON internet providers.