What I do is I create a folder and will name it 'a', 'b', 'c' and so on.
In a folder there are 4 to 5 different set of categories. After showing all of the categories for 6times, then I rename the folder RET a.
Now I'm on RET 2d. Which means that my son has already seen RET a to RET z folders for 6times, also he has already seen RET 2a to RET 2c folders, 6times.
With regards to how many times you will show a category or categories to your baby depends upon you. I decided to show my son 6times since I find him bored when I show him those categories more than 6. And you know, my goal is not for him to memorize what are in those categories but just to show him all the facts in those categories. I think 6 times for my son is enough and I don't need to go for 30times I think. As long as he is happy seeing it, then I continue. I always believe that my son is learning. And really, he is learning. He knows so many for a boy who is 2 years old.
But the thing is, it's really up to you. Mothers knows best for her child.
I don't download categories until my son is finished with the current folder. With this, I keep my library organized and what you will see are just folders.