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It's hard to assess exactly 'how long' until the polyps will extend. Some will extend polys right away while others may take a day or two. Were the polyps extended prior to you getting it?
Did the water it was previously kept in differ much from your tank's parameters? Did you acclimate the coral at all?
I usually at least test the SG from the water at the LFS when I get anything. It could be the flow, light or it is just really pissed off at something else.