It takes about 0.15 seconds from the moment light hits the retina to when the earliest recognition of basic object identity can occur. [1]

The fastest the brain can process successive unrelated images is if they are presented about 0.1 seconds apart. [2] At that speed, two images would be present in the visual processing "pipeline" at different stages simultaneously.

This diagram shows the time elapsed in milliseconds (left) from the time the image hits the retina (at 0 ms) to when the signal arrives at successive brain areas in the visual system (retina -> LGN -> V1 -> V2 -> V4 -> PIT -> ...). The middle triangles show the size of the visual area handled by neurons, and the right side depicts the level of pattern recognized by each area:

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[1] Thorpe SJ, et al (1996). Speed of processing in the human visual system. Nature. (http://scholar.google.com/scholar?cluster=5903077898808176505)

[2] Gerson AD et al (2006). Cortically coupled computer vision for rapid image search. IEEE Transactions on Neural Systems and Rehabilitation Engineering. (http://scholar.google.com/scholar?cluster=16988215059538861796)