FYI I've never seen anything written about the length of time PURE LOCAL ranking penalties last other than my own comments on the topic.
But in ranking troubleshooting/consulting cases I've worked on, that were lifted within a week after I pointed out the offending problem and it was fixed. I've also seen penalties however that lasted 8 months.
In the post below there is a quote from Matt Cutts talking about how the severity of the penalty fits the crime. I assume that's also true in local.
See it on Scoop.it, via Google+ Local & Local SEO News
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