off the top of my head, I believe there's a minimum size for current cocks to be at, to grow a new one with an incubi draft. I forget if it has to be unpurified or not. I remember fighting like hell to grow three tentacle dicks for the forest kitsune special scenes, including pumping my corruption up to like 70 in the process, although that may have just been for the corrupted glade? Not sure if that has any effect for the dick growing, it would depend on if that's considered a demonic mutation, which happens more frequently when you have higher corruption.
I do vaguely remember save-scumming for a very long time with incubi drafts though. I'm afraid I don't remember if that was for just the 2nd or the 3rd dick.