I made the transition from a Thinkpad keyboard to an ErgoDox 3-ish years ago. For me I'd say it took about 2 weeks to be fully back to normal with it, and about a week to stop feeling like I couldn't type.

In my case, I had some bad habits I was needing or wanting to get rid of at the same time: I typed "b" with the wrong finger, I always used my right pinky for shift, I always looked when typing numbers so I never really got to touch-typing them, sometimes I would also peek for regular letters or keys, just to make sure I was where I thought I was.

If you have fewer bad habits, it might take less time. If you have more... :-)