Probably pretty low. When it comes to breeding it takes lots of time to breed out specific traits of a strain, and can take many generations to get there. If you only have a few seeds you would need to be ridiculously lucky that by complete chance that the one time you cross them you end up with the auto flowering trait. If you want to breed for traits I recommend heading over to the breeding subforum and read up. Basically first you would have to grow many of the 2 plants, when you cross breed them plant and let those seeds grow out. From there you would need to see which ones have the auto flowering traits. Then considering if you don't care what the rest of the plants attributes are you could call it done, but if you are interested in selecting more specific traits to it (taste, color, THC level) you would need to continue breeding and narrowing it down until you have what you want.
I'm pretty sure that is how the process works, but I am not a breeder. All I know is it is not as easy as cross pollinating them once and calling it a day.