I'll address the question in the title "At which time did sight evolve for the first time?" by assuming that by the evolution of vision, we mean the evolution of the eye.
Molluscs are an excellent phylum to investigate this question because they exhibit a wide range of eye designs and levels of complexity.
At the most basic level, limpets such as Patella exhibit small patches of photoreceptor cells lying in a relatively flat configuration. Slightly more advanced is Pleurotomaria which has photoreceptors and pigmentation cells held in an eyecup. We then have the pinhole camera style eye as seen in Nautilus (see this post), and more complex eyes with a cornea, retina, and lense such as those seen in squid (e.g. Loligo).
Assuming that "a patch of photoreceptors" in an animal counts as an eye, then we should probably looking for marine invertebrates. The problem here is that if they had only soft body parts we might struggle to identify the oldest examples in sediments.
One candidate for the "earliest eye" might be urbilatarians - the hypothesized last common ancestor of the clade bilatarians - which probably evolved at the end of the Ediacaran period (~555 Myr). An example would be Kimberella (described here) which might or might not have been a mollusc and might or might not have had photoreceptors!
(credit : wikipedia)
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