Had the proof-of-concept (working audio sequencer in the browser) working in a few days, and a full prototype to share with my students in a month. It was the definition of hacked together, but it was usable for practicing the songs I'd pre-programmed into it.
My students loved it. I loved playing along with it too. I was pumped. This was not just going to be something for my students. I was going to get the wider music community using it too.