", it means something more like "be away for this length of time". For example, I would naturally say "I'm going to the supermarket; I'll be half an hour". To me, this is equivalent to "I'm going to the supermarket; I'll be THERE FOR half an hour" or "I'll be AWAY FROM YOU FOR half an hour", or "I'll be BACK IN half an hour".
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MUSCOVITEThis phrase looks to me like a contracted form of another ('full-fledged') expression.Right. It is.