Extended similes are, not surprisingly, similes which are continued at length. They are known as heroic similes in the classical tradition, where they were often used in poems about heroes.
They were much admired in classical writing (in Latin and Greek, and in English by those who admired the ancient styles). They often begin, in neo-classical English verse, with the words "As when ..." (~ "This is like the time[s] when ..."). There is a whole sonnet of 14 lines by John Codrington Bampfylde ("As when, to one who long hath watched...", available on the web, for example, on 7th November 2006 at http://www.theotherpages.org/poems/2001/bampfylde0101.html), in which the first 11 lines describe someone watching the dawn break. Only the last three lines come to the point: "Such is the sight of thee" - that seeing the sun rise is, for the poet, like seeing his beloved.