As of today, it has been one month since I started using the Twiddler.
I've logged almost 25 hours in the tutor at this point, though I spent the first six hours just on ETAOIN before realizing you can override lesson complexity. My current daily average is 37wpm. This is, of course, somewhat exaggerated--on https://www.livechat.com/typing-speed-test/#/ I average about 25wpm, and with punctuation and capitalization I drop down to 22wpm.
My configuration was, by design, extremely simple to memorize (it's based off of Cool Hand and Typemax,) and muscle memory really kicked in around the 15-hour mark. Right now my main roadblock is finger fatigue—after 1.5 hours or so my speed and accuracy dip considerably, especially for letters that use the pinky.
I suspect my wpm will continue to increase over the next several months, though I doubt I'll ever reach @lancegatlin's speeds with Typemax. Still, considering that my typing speed on a keyboard with two hands is 60wpm, I'm perfectly happy with my progress--I'll probably end up with over half of my keyboard speed, which is better than I expected.