I'm not sure. From July 2006 to April 2015 I didn't pay attention to my subscriber count. According to Analytics, which says subscriber count wasn't tracked correctly before September 23, 2009, I first reached 100 subscribers on January 19, 2015. I was just uploading videos for fun, whenever I felt like it, sometimes a month between uploads, and barely even looking at views or subs.
Then one of my videos started getting a ton of views. It had 54,000 views at the beginning of April, and reached 1,000,000 views by April 18, which was also when I hit 1,000 subs. It was at that point I enabled monetization and started making videos regularly to try to keep it going.