'As <noun-plural> go' is an informal way of saying 'in comparison with others'. In this case, other recessions had been longer than the one in 1907.'
This idiom was [ab]used by Saki in 1904: 'The cook was a good cook, as cooks go; and as cooks go she went' - in which the second 'go' (as the 'went' suggests) really means GO. (Read more about him here; he's 'worth a visit': Saki - Wikipedia, the free encyclopedia )
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