For me, i didn't find any difficulty at all. I typed fast on it the first time i touched it, at some 50 wpm. (my normal speed is 80 wpm)
This is probably because i used Truly Ergonomic keyboard (TEK) for 3 years before. TEK has straight aligned keys. When i used TEK the first time, it did took me about 3 weeks to become fluent. I kept making some errors on qwerty key b.
But the bowl surface is first for me, still, i was surprised i was comfortable right away as if didn't even notice it's bowl shaped.
if i were to guess from my experience, for a normal PC keyboard user, you might take 1 month to be fully fluent as before. It took me about that time when i used TEK.