Tim V of Wine making Magazine is starting to say that a kit wine should stay in secondary at least the number of days recommended in the instructions, regardless if the SG stops dropping early or not.
I have no reason to doubt his wisdom. If that's what you want to do, here's how I would proceed - For example, if the instructions say the wine should stay in secondary for 8 days, I would check the SG on day 6, seal the wine up again, then check it again at the end of the 8th day.
If the SG is the same on both days, the wine is finished fermenting, so I would move onto the next step. If the SG has fallen some from day 6, I would wait another two days and check it again until it has stayed the same for three days in a row.
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