What is the most accepted time for a pre-tenure move?

I think the answer is "when you find a good opportunity." I have a slightly embarrassing amount of experience in this regard, as well as some experience on the other end of things viewing such applications. My experience is that they will not care where you are in your tenure review process; the advice about moving pre- vs. post-tenure is mostly just because there are usually more internal barriers to hiring someone with tenure (which still happens quite often).

There’s no such thing as a normal or typical time to move. Faculty move when the right opportunity presents itself. So long as you’re upfront with everyone, I wouldn’t worry about the timing. The only caveat is to try not to leave your old department in the lurch when you go—if possible, give them enough notice that they don’t have to scramble to replace your teaching load and other assignments on short notice.

Note that those arrangements could be through adjuncts or reassigning the load among the existing faculty, or anything else. I agree waiting for a TT hire is unreasonable—the process just takes too long. However, if you've been there a year or two, you should already have a sense of when teaching assignments are being decided for the following year and can plan accordingly.