'Not longer than' is acceptable here, as is 'no longer than'. Actually, if this is part of a contract, it's more likely to be something along the lines of '... when they are outside your flat for a perid not exceeding three (calendar) months' (or, 'ninety days'.)Here's the context. The sentence I am trying to correct is:
'Your contents are insured when they are outside your flat not longer than three months.'
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