an F2 cross is likely to produce unstable results. Some will be autoflowering some will not. If you continue to breed the autoflowering ones, chossing only the autoflowing individuals, you could produce a true breeding strain after atleast 7 generations, only problem here is your plant probably wont have the hybrid vigor it once posessed. You could Back cross your F2 generation to the mother , and take the resulting seeds and cross it again to the mother, every time you do this the % of the mothers genetics increases.
If you could find pure ruderalis seeds, they are true breeding and autoflowering, although rather unpotent, you could make your own ruderalis cross potentially resulting in something rather nice, and autoflowering. your cross wont be true breeding, but you will have both the mother and father to produce auto flowering seeds.
Im not sure whether mother or father ruderalis better pass on the autoflowering trait. I doubt that they would want to go that far into revealing their secrets.