Hi I think you are right to raise that question. My feeling is that we don't often use that tense in everyday speech. When we do use it, it tends to be in sentences that we've thought out before saying or writing them - If you don't get here in the next ten minutes, I'll have been reading a very boring newspaper for more than an hour - By the time I retire, I'll have been a qualified accountant for thirty years We do use it to sympathise with another person: - I'm exhausted.
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