What's the shortest "acceptable" postdoc length? So, the postdoc is going ok and there might be one or two publications after a few months. However, I've realized that I'd like to move forward and start applying to industry jobs. 

Now, my concern is whether having a few-month postdoc in the resume/CV might hurt, and I wonder what would be the minimal "ok" length for a postdoc. I have the feeling that an ultra-short postdoc might seem suspicious to a recruiter (it may look like I was ultra-bad and was kicked out, which is not the case). 

While some people would claim "leave the postdoc ASAP, especially if you want to go to industry", I was wondering whether something around 6-8 months is fine..., or maybe I should wait >= 1 year.
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.