I can't really say definitively how long it should take ... we all have differently shaped learning curves, after all. I know some people who you can show something to who will go out and do it almost reflexively right away. Others need multiple attempts to get it right the first time, and lots of practice to become proficient at it. Something like a regulator switch shouldn't be that difficult, since we all learned how to accept a safe second from our dive buddy during our OW class, and hopefully practiced the skill somewhat thereafter.
I took my initial sidemount instruction in a cave. Now, granted I was already cave certified, and I had plenty of practice managing multiple regulators as a tech diver before I took the sidemount class. But it would've been impossible if my instructor had insisted that I become proficient with diving sidemount prior to taking the class.
I get that solo diving is a bit unique, in that you won't have someone there to cover your mistakes if you should make them. But I just don't think that managing a second regulator is all that difficult. For most folks I think a bit of dryland practice and a dive with a handful of repetitions should get you to the point where you can manage it in an emergency.
... Bob (Grateful Diver)
