It would take at least 5 generations imo. I have crossed auto females with regular pollen and the offspring show traits of, #1 mother, #2 the father, and #3 a mix of both of them. You would have to select the phenos which showed the trait that you want to keep. so say you grow 30 of them and cross with a regular, 10 will be straight auto, 10 will be straight regular photoperiod, and 10 will have the tendency to auto flower, maybe 16hrs of light will flower them. (this is a general idea, nothing scientific here just my experience with autos)
Now once you have grown them for at least 45 days do your first selection. Find one that does not flower under 24 hrs light. That selected pheno will have the auto trait still locked in her genes. Breed her with a regular strain for the second time. Again each time you cross regular into it the less of the auto flower trait it will have. grow 30 seeds again, 24hrs light, find one that doesnt flower under 24 hrs light within 60 days just to make sure the regular trait is dominate. Select the 3rd pheno, breed it with a regular again, same deal, grow 30 seeds, but this time dont run 24hr light, and find the one that doesnt auto flower under 18 hrs light. Now the last selection #4, you should grow them under 15 hrs light and see which one doesnt auto flower. Time for the back cross to stabilize the regular trait, take the original pollen, or original seeds from the "first" cross and back cross it into the 4th pheno. Your off spring should now have close to 90% regular dominate genes. so out of 100 seeds maybe 5 or 10 may auto flower, but its unlikely. And as far as every single auto flower plant being weak, is a myth. A good deal of autos will be weak and wear off quickly, but there is a percentage of them that will knock you on your ass!!
This is how I would do it. Im not saying im 110% right and you have to do it this way, this is just my opinion of how you could breed out the auto flower trait. There may be a faster way to do it in like 3 generations, I have studied autos and how they make them, so to "undo" the auto trait I would think you would reverse the process of how they make them. Im also not saying im better than everyone!! good luck man!!
And just to thro this in, I grew 5 generations of auto flowers from may to September to give you a time line close to what it would take,,,,, IMO!!