There is no single answer to your question, but a truck load of conditional answers depending on many factors.
Go here, and look up the lengths(on the left), then put in your particulars for an idea of stability.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
One of the many reasons the answer depends, is barrel smoothness plays a HUGE role in whether or not something actually stabilizes; regardless of the calculator.
As far as "Pushing the top end". That also depends. Since the days of WWII have long past, no one lists brass alloy, nor heat treat specs on brass anymore. So if you want to stay within SAAMI pressures, book data is ALL pressure tested. The loads vary simply due to burning rate variation with powders. In any case, an extra 75-100 fps will cost you something on the order of 15,000 psi additional. Without knowing your chamber and brass composition, NO ONE can tell you when the brass will fail.
From using a Pressure Trace over the holidays 8 months ago, MOST production firearms won't show classic "pressure signs" until you are well into the 70,000+ psi range. So short of a good Strain Gauge system to honestly measure pressure, you won't be comparing apples to apples.
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