Stephen, I am not sure you intended to paint with such a broad stroke. You are correct for this gentleman pursuing this full time, but your response could be misconstrued to suggest that all flips lose capital gains treatment. This answer could be different on other "flips." A flip (as we call it) is not inherently self employment income, although following this standard rule of thumb is generally the least subject to IRS scrutiny (as it is the most punitive) as everyone sees a rule of thumb as "the law," even when it is far from. A full-blown construction operation where you are flipping houses as the core business is clearly not an investment, it is a business. For the rest of us trying to build wealth, there are situations where a quick sale of a property you just renovated was very much an investment. For this sort of investor, it is a good practice before/at acquisition to document your investment intent (and do so in good faith). This intent does not mandate a "holding period" similarly as I have had many-a-short-term gains/(losses) in the stock market and no one argues that I was running a moderately unsuccessful trading business. The holding period really only impacts whether the gain is short term or long term, but neither get taxed as self employment. The IRS has a hard time (never succeeding as I understand) challenging your investment intent when you clearly documented this at inception (date it and get it notorized). If you periodically, even regularly, monetize these investments to raise capital for other similar investments, that is completely legitimate. It is not entirely dissimilar to swapping out of one stock that has just popped (leaving little remaining upside in you view) and into another that has a better risk-reward profile. Why would you hold it if you had a better alternative. This is the case on most (not all) flips as you create the most value after rehab, a great investment. The rationale at that time is whether the future value generated by holding the investment warrants keeping it or whether you have other investment opportunities that require you to liquidate the investment. Your choice to liquidate the investment is to generate capital needed for another investment and that is where the cash should go. This approach will impact your flexibility as you should really be committed to these investment decisions, as I would formalize each of these decisions and have them notorized.
For those of you who don't flip to live, but rather to build wealth, this is something to consider and talk to a knowledgeable tax attorney who specialize in real estate (these are rare and sadly rarely hang out in forums).
Just some food for thought.