Ok, my answer of reading the POH was a little flippant, and IO540 is correct to say it's not quite so simple.
However, it's a good place to start. TotalBeginner has read his POH, and told us that it does not recommend leaning below 3000'. The answer to his question, therefore, is to not lean below 3000'.
Whilst there are lots of debatable areas when it comes to leaning (rich or lean of peak, leaning for take-off, etc, etc, etc), and the instrumentation you've got available to you might well be a big factor in answering these questions, I would have thought that there can be little doubt that if the POH says don't lean, you don't lean???
FFF
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