For me personally I could start to clearly see a pattern at about 10 months. However one just cannot know how much the hair will shrink after the straight ends are gone, and shrinkage of course changes the look of the hair. But you can figure out other things, a year in I knew where the curl would be weakest (the front, bang area), what section was the kinkiest (just under the crown), what was the coiliest (the temples) etc. I transitioned for 28 months.
I tend to think that it's not a bad idea to BC tiny sections in different areas. I did this and it definitely helped me to have patience, as I already knew the pattern, and I practiced a little in these areas too.
Edit: Ah I just remembered that I have a transition folder
here, showing the same area (more or less) every few months if you're curious. Those pics were for the most part taken on drenched hair, and looking back on them now, my pattern when drenched is really not that different from what the pics show. But as it dries and shrinkage kicks in, it'll look poofier, shorter, bigger, etc. Products, styling and length can really make the same head look completely different, I find!