yea, it takes some time. I have 64 ''recipes'' now, but I ain't doing this very long. This month I didn't do any card, cause I wasn't in the right mood. But I have days, when I wanna find one color and I make 3 cards fast. I scuff like 10 of them in a go, and they are waiting in my airbrush section for spraying session.
How useful those ''recipes'' are, probably depend of individual. I can't eyeball colors like many of forum members here
For example: I have unopened grey from wicked which I will spray on a card, so I can then find the ''recipe'' of that gray and mix it by myself, cause I tend to buy primary colors in bulk. It is cheaper on long run.