Not nearly a decade. There are great resources that start you with just a few keys and add more over time. Once you have all 26 letters, it’s time to move on to MonkeyType or an equivalent. You will absolutely have reduced output for a few weeks. I notice a huge difference compared with QWERTY when typing on a MacBook all day. You should not switch layouts for speed, but rather for comfort. The GitHub link contains a few interesting graphics about efficiency if you’re interested, but the very high-level result is sizable comfort gains.

Probably around one month. Using Colemak Mod-DH on an external keyboard and QWERTY on my Macbook. My brain is switching automatically, cannot use them in a reverse order.