One Coat Plaster

I've used some one coat plaster to repair a wall ready for tiling. There is an area at the bottom which is not drying out fully after 3 days. It's feels solid but is darker than the rest of the area. It's probably the deepest area at about 20mm so how long should it take to dry?
 
It will be dependent on lots of things, the suction of the substrate, temperature, depth etc to give you a specific answer. You will be able to see when it is dry due to the colour change.
 
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