Thirty minutes, made possible by the fact that the main quest has few areas where you actually need to kill your adversaries (perhaps nothing beyond clearing Project Purity of enemies for your father) and you could just run past them.
That and several segments of the main quest can be skipped.
EDIT: Not sure if my memory is %100, but I believe you can go after your father in the simulation even if you didn't look for him at Rivet City.