What's the shortest "acceptable" postdoc length?

If you know that you are not interested in staying in academia, the common advice is indeed to go on the industrial job market as soon as possible.

Every month you stay in a lowly-paid postdoc, if you are not enjoying it and are not building an academic profile, is essentially wasted.

Your question is based on the somewhat suspicious premise that an industrial recruiter cares about how long you stayed in your postdoc, i.e., that your market value goes up by staying in a postdoc. I highly doubt that this is the case, but you can easily evaluate it yourself: go on the job market right now (without quitting your postdoc!) and see if you get offers that you find acceptable. If you do not (for whatever reason), you can still continue your postdoc and improve your market value on the side; if you do, well, quit within an appropriate time frame and start your new life.

Note: this advice is based on the assumption that you can delay the start date of your industrial job by a few months so that you can leave your postdoc in order. In my experience, this is often the case for the kind of jobs that PhD holders tend to look for, but obviously your milage may vary on this.

@JessicaB's comment is right:

Some postdoc positions may only be 6 months long (I think 3 has occurred but I can't remember exactly).

I've seen a few people recruited for 6 months, and applied for one less than a year long myself.

Don't go into detail about why you left unless asked. Being thrown out of a job is rare in most places, especially academia, so this probably won't even be on the recruiter's radar. They're more likely think either "no wonder they want to leave academia with that kind of job security" or "this applicant is going to jump on every better opportunity that comes their way". Your task is to steer them towards the former view.

If your goal is to go to industry, every additional month that you stay in your postdoc is costing you both money and valuable work experience. Money, because industry jobs pay better than postdocs. Experience, because a month of industry experience is worth more for a career in industry that a month of postdoc experience.

In other words, the "shortest acceptable postsoc length" is zero.