I did the green book (McDowell) cover to cover, with a few dedicated hours of study each day, and random googling/tinkering where I wanted to explore more. I won't say it was the most fun I've ever had, or as self-motivating as a nice side project, but it was a good change of pace over gluing APIs together, so the novelty effect kept me going for a while. I probably put 150 hours of prep in over 6-8 weeks. The ROI on that time (relative to competing startup offers) was very far north of $1k/hour.