Start to finish... a long, long time. 18 months on and off, 4 months full time. I started this before I was employed at PlanetScale and when I joined PS the course came along with me. I wouldn't have had the space to do it if it hadn't been my full time job. For the past four months or so I've been spending 100% of my work time on it, and much of my spare time.
Part of what made it tough is that, while I've been comfortable with MySQL for a long time, _teaching_ it is a whole different thing. So I ended up having to study a _ton_. Lots and lots of videos trashed when I got to a point of explaining something and realized I didn't know it well enough to teach. Back to the docs to figure it out myself and then back to recording.
I could do it again in about 1/3rd the time, but of course I could! I've done all the hard parts now! The actual recording and editing was of course hard, but the up front work to make sure I wasn't making stuff up was probably the biggest slog of it all.