Note that Off-Peak times are not fixed across all journeys or operators - they depend on the exact fare you have bought. As
@jfollows has quoted, it can even vary between standard and first class versions of the equivalent fare.
It's best to refer to the restriction code associated with the ticket you're going to buy. This should generally be indicated when you buy your ticket. In any event, you will only be offered Off-Peak fares for trains on which they are valid.